vshExtractCPUDefXML: Accept domain capabilities XML

vshExtractCPUDefXML: Accept domain capabilities XML

The domain capabilities XML contains host CPU model tailored to a
specific hypervisor and since it's enclosed in <mode name='host-model'>
element rather then the required <cpu> it's impossible to directly use
the host CPU model as an input to, e.g., cpu-compare command. To make
this more convenient, vshExtractCPUDefXML now accepts full domain
capabilities XML and automatically transforms the host CPU models into
the form accepted by libvirt APIs.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Collin Walling <walling@linux.ibm.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>