Probe QEMU directly for machine aliases if not found in capabilties

Probe QEMU directly for machine aliases if not found in capabilties

Not all possible emulators are actually in the capabilities, so if we
don't find the supplied emulator we should probe it directly for machine
types.

* src/qemu_driver.c: add qemudCanonicalizeMachineDirect() to directly
  probe an emulator for the canonical machine type