When migration fails in the post-copy mode, it's impossible to just kill
the destination domain and resume the source since the source no longer
contains current guest state. Let's mark domains on both sides as
VIR_DOMAIN_PAUSED_POSTCOPY_FAILED to let the upper layer decide what to
do with them.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>