Postcopy: Reset state to avoid cleanup assert

On a destination host with no userfault support an incoming
postcopy would cause the state to enter ADVISE before
it realised there was no support, and because it was in ADVISE
state it would perform a cleanup at the end.  Since there
was no support the cleanup function should be unreachable,
but ends up being called and asserting.

Reset the state when we realise we have no support, thus the
cleanup doesn't happen.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20170202155909.31784-2-dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>