migration: disallow recovery for release-ram

Postcopy recovery won't work well with release-ram capability since
release-ram will drop the page buffer as long as the page is put into
the send buffer.  So if there is a network failure happened, any page
buffers that have not yet reached the destination VM but have already
been sent from the source VM will be lost forever.  Let's refuse the
client from resuming such a postcopy migration.  Luckily release-ram was
designed to only be used when src and destination VMs are on the same
host, so it should be fine.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180723123305.24792-3-peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>