domcaps: Check for architecture more wisely
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1209948
    
    So we have this bug. The virConnectGetDomainCapabilities() API
    performs a couple of checks before it produces any result. One of
    the checks is if the architecture requested by user can be run by
    the binary (again user provided). However, the check is pretty
    dumb. It merely compares if the default binary architecture
    matches the one provided by user. However, a qemu binary can run
    multiple architectures. For instance: qemu-system-ppc64 can run:
    ppc, ppcle, ppc64, ppc64le and ppcemb. The default is ppc64, so
    if user requested something else, like ppc64le, the check would
    have failed without obvious reason.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>