domcaps: Add CPU usable flag
    
    In case a hypervisor is able to tell us a list of supported CPU models
    and whether each CPU models can be used on the current host, we can
    propagate this to domain capabilities. This is a better alternative
    to calling virConnectCompareCPU for each supported CPU model.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>