qemu: Ignore fallback CPU attribute on reconnect
    
    When reconnecting to a running domain with host-model CPU started by old
    libvirt which did not store the actual CPU in the status XML, we need to
    ignore the fallback attribute to make sure we can translate the detected
    host CPU model to a model which is supported by the running QEMU.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1532980
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>