tests/migration-tests: Cover postcopy failure on reconnect

tests/migration-tests: Cover postcopy failure on reconnect

Make sure there will be an event for postcopy recovery, irrelevant of
whether the reconnect will success, or when the failure happens.

The added new case is to fail early in postcopy recovery, in which case it
didn't even reach RECOVER stage on src (and in real life it'll be the same
to dest, but the test case is just slightly more involved due to the dual
socketpair setup).

To do that, rename the postcopy_recovery_test_fail to reflect either stage
to fail, instead of a boolean.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>