domain_capabilities: Add vendor attribute for CPU models

domain_capabilities: Add vendor attribute for CPU models

Even though several CPU models from various vendors are reported as
usable on a given host, user may still want to use only those that match
the host vendor. Currently the only place where users can check the
vendor of each CPU model is our CPU map, which is considered internal
and users should not really be using it directly. So to allow for such
filtering we now advertise the vendor of each CPU model in domain
capabilities.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>