schema: Allow counter element in host cpu definition
schema: Allow counter element in host cpu definition

If the capabilities include a counter element, e.g.
  <counter name='tsc' frequency='2591999000' scaling='no'/>
the XML could not be validated:
  $ virsh capabilities > cap.xml
  $ virsh [hypervisor-]cpu-compare cap.xml --validate
  error: Failed to compare hypervisor CPU with cap.txt
  error: XML document failed to validate against schema: Unable to validate doc against /usr/share/libvirt/schemas/cpu.rng
  Did not expect element counter there

Signed-off-by: Tim Wiederhake <da93ee425e056cfcf1ddac9fc87851b25464abf4@redhat.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

jfehlig/libvirt,olafhering/libvirt,olafhering/libvirt,olafhering/libvirt,crobinso/libvirt,zippy2/libvirt,jfehlig/libvirt,zippy2/libvirt,libvirt/libvirt,nertpinx/libvirt,nertpinx/libvirt,crobinso/libvirt,jfehlig/libvirt,nertpinx/libvirt,crobinso/libvirt,jfehlig/libvirt,nertpinx/libvirt,nertpinx/libvirt,olafhering/libvirt,crobinso/libvirt,libvirt/libvirt,zippy2/libvirt,libvirt/libvirt,libvirt/libvirt,zippy2/libvirt