virsh-domain: Mark --live and --config mutually exclusive in vcpucount
    
    The 'vcpucount' command is a getter command for the vCPUu count. When
    one or more of the filtering flags are specified the command returns the
    value only for the selected combination. In this case the --live and
    --config combination isn't valid. This however didn't cause errors as
    the combination of flags was rejected by the libvirt API but then the
    fallback code kicked in and requested the count in a way where the clash
    of the flags didn't matter.
    
    Mark the flag combination mutually exclusive so that users aren't
    confused.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1024245