qemu: Fix regression when hyperv/vendor_id feature is used

qemu: Fix regression when hyperv/vendor_id feature is used

qemuProcessVerifyHypervFeatures is supposed to check whether all
requested hyperv features were actually honored by QEMU/KVM. This is
done by checking the corresponding CPUID bits reported by the virtual
CPU. In other words, it doesn't work for string properties, such as
VIR_DOMAIN_HYPERV_VENDOR_ID (there is no CPUID bit we could check). We
could theoretically check all 96 bits corresponding to the vendor
string, but luckily we don't have to check the feature at all. If QEMU
is too old to support hyperv features, the domain won't even start.
Otherwise, it is always supported.

Without this patch, libvirt refuses to start a domain which contains

  <features>
    <hyperv>
      <vendor_id state='on' value='...'/>
    </hyperv>
  </features>

reporting internal error: "unknown CPU feature __kvm_hv_vendor_id.

This regression was introduced by commit v3.1.0-186-ge9dbe7011, which
(by fixing the virCPUDataCheckFeature condition in
qemuProcessVerifyHypervFeatures) revealed an old bug in the feature
verification code. It's been there ever since the verification was
implemented by commit v1.3.3-rc1-5-g95bbe4bf5, which effectively did not
check VIR_DOMAIN_HYPERV_VENDOR_ID at all.

https://bugzilla.redhat.com/show_bug.cgi?id=1439424

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>