When there is IO error on the incoming channel (e.g., network down),
instead of bailing out immediately, we allow the dst vm to switch to the
new POSTCOPY_PAUSE state. Currently it is still simple - it waits the
new semaphore, until someone poke it for another attempt.

One note is that here on ram loading thread we cannot detect the
POSTCOPY_ACTIVE state, but we need to detect the more specific
POSTCOPY_INCOMING_RUNNING state, to make sure we have already loaded all
the device states.
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NPeter Xu <peterx@redhat.com>
Message-Id: <20180502104740.12123-5-peterx@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>