This will allow us to more easily filter and combine CPU models with the
multiple test cases we have. We can easily introduce new tests that will
automatically be run once for each known CPU model.

Test cases that don't need to be re-run for each CPU model can use the
"cpu.unset" variant that doesn't set any specific cpu_model option.

This also introduces "kvm" and "nokvm" variants, to allow test cases to
be run with KVM disabled and enabled, if necessary.

This way the test output will now look like this:

    (1/94) qemu_cpu.check_models.kvm.cpu.intel.486.qemu13: PASS (0.42 s)
    (2/94) qemu_cpu.check_models.kvm.cpu.intel.core2duo.qemu13: PASS (0.44 s)
    [...]
    (49/94) qemu_cpu.cpuid.default.vendor.nokvm.cpu.intel.core2duo.qemu13: PASS (20.63 s)
    (50/94) qemu_cpu.cpuid.default.vendor.nokvm.cpu.intel.kvm32.qemu13: PASS (21.66 s)
    [...]
    (62/94) qemu_cpu.cpuid.default.vendor.nokvm.cpu.intel.SandyBridge.qemu13: PASS (20.58 s)
    (63/94) qemu_cpu.cpuid.default.vendor.nokvm.cpu.intel.Haswell.qemu13: PASS (20.52 s)
    (64/94) qemu_cpu.cpuid.default.vendor.nokvm.cpu.amd.qemu64.qemu13: PASS (21.51 s)
    (65/94) qemu_cpu.cpuid.default.vendor.nokvm.cpu.amd.phenom.qemu13: PASS (20.53 s)
    [...]
    (71/94) qemu_cpu.cpuid.default.vendor.nokvm.cpu.amd.Opteron_G5.qemu13: PASS (21.59 s)
    (72/94) qemu_cpu.cpuid.custom.vendor.normal.kvm.cpu.unset.qemu13: PASS (20.53 s)
    (73/94) qemu_cpu.cpuid.custom.vendor.tooshort.kvm.cpu.unset.qemu13: PASS (0.87 s)
    (74/94) qemu_cpu.cpuid.custom.vendor.empty.kvm.cpu.unset.qemu13: PASS (0.81 s)
    (75/94) qemu_cpu.cpuid.custom.level.level5.kvm.cpu.unset.qemu13: PASS (20.60 s)
    [...]
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>