A microcode update can cause the CPUID bits to change; an example
from the past was the update that disabled TSX on several Haswell
and Broadwell machines.

Therefore, place microcode version in the virQEMUCaps struct and
XML, and rebuild the cache if the versions do not match.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>