Features/qtest driver framework

Through a driver framework, libqos can expose a description of QEMU's supported machine types and a set of drivers; unit tests can request a driver, and the framework takes care of starting QEMU with options that provide that driver.

For example, the framework could provide:

1. an interface for SD-HCI devices (abstract class QSDHCI)
2. a driver for a PCI SD Host Controller Interface device (a subclass of QSDHCI, for example QSDHCI_PCI)
3. a description of how QEMU's "sdhci-pci" device maps to QSDHCI_PCI
4. an interface for a PCI bus (abstract class QPCIBus)
5. a driver for the PCI bus in an x86 PC machine (a subclass of QPCIBus, for example QPCIBusPC)
6. a description of QEMU's "pc" machine (a subclass of QOSMachine) and its embedded devices (in this case a QPCIBusPC)

(Right now libqos provides items 4 and 5 only).

You can construct a graph where the nodes are interfaces, drivers and unit tests, connected by relations such as "X produces Y" or "X consumes Y":

 (driver)           (driver)           (interface)        (interface)        (driver)
     pc  ────────→ i440FX-host ───────→ QPCIBusPC  ───────→ QPCIBus ───────→ QSDHCIPCI
          produces             produces              is-a           consumes     │ produces
                                                                                 ↓
                                                                            (interface)
                                                                              QSDHCI
                                                                                 │ consumes
                                                                                 ↓
                                                                             register-test

(The above is just a single path in the huge graph for the "x86_64" QEMU target architecture!). For example, a unit test for the SD-HCI device (let's call it "registers-test") would add itself to the graph like this:

 qos_add_test("register-test", "sdhci");
                    │             ╰────── Interface consumed by the test
                    ╰──────────────────── Test name

After the graph is build, tests can be discovered by walking the graph: each test to run corresponds to a path from the root of the graphs to a unit test. The above path could represent a test like "/x86_64/pc/i440FX-host/pcibus-pci/sdhci-pci/sdhci/registers-test":

  /x86_64-softmmu/pc/i440FX-host/pcibus-pc/sdhci-pci/sdhci/registers-test
        │         │       │          │         │        │       ╰──── Test name
        │         │       │          │         │        ╰──────────── Driver or interface provided by "sdhci-pci"
        │         │       │          │         ╰───────────────────── Device on the PCI bus
        │         │       │          ╰─────────────────────────────── Driver or interface provided by "i440FX-host"
        │         │       ╰────────────────────────────────────────── Device embedded by "pc"
        │         ╰────────────────────────────────────────────────── QEMU machine
        ╰──────────────────────────────────────────────────────────── Target architecture

At run-time, a lot of steps are hidden in the framework, and are realized by walking the path on the graph:

• QOSMachine walks the path to the device under test
  • the machine itself adds "-M pc" to the command line
  • QPCIBusPC need not add anything to the command line because the device is embedded
  • QSDHCI_PCI adds "-device sdhci-pci" to the command line
• QOSMachine starts QEMU
• QOSMachine finally executes the code for the unit test
  • The test asks QSDHCI_PCI to start the device
    • QSDHCI_PCI sets up the PCI device using the methods of QPCIBus
  • The test runs and tests the SDHCI device