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>

tests/cputestdata/x86_64-cpuid-Core-i7-3770-host.xml

@@ -2,6 +2,7 @@
   <arch>x86_64</arch>
   <model>IvyBridge</model>
   <vendor>Intel</vendor>
+  <signature family='6' model='58' stepping='9'/>
   <feature name='ds'/>
   <feature name='acpi'/>
   <feature name='ss'/>