conf: cpu: Add <signature family=X model=X stepping=X/>

conf: cpu: Add <signature family=X model=X stepping=X/>

Internally we already collect x86 host family + model + stepping
numeric values. This exposed them in capabilities CPU output.
Example:

$ sudo virsh capabilities | grep -A1 -B1 signature
      <microcode version='240'/>
      <signature family='6' model='94' stepping='3'/>
      <counter name='tsc' frequency='3408010000' scaling='no'/>

Users need to know these values to calculate an expected.
SEV-ES/SEV-SNP launch measurement.

Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>