qemuConnectGetDomainCapabilities: Report error on unknown arch
    
    If user hasn't provided any @emulatorbin, the qemuCaps are
    searched by @arch provided (which in fact can be guessed from the
    host). However, there's no guarantee that the qemu binary for
    @arch will exist.  Therefore qemu capabilities may be nonexistent
    too. If that's the case, we should throw an error message prior
    jumping onto 'cleanup' label as the helper lookup function
    remains silent on no search result.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>