Workaround fake failures in drbd+live migration

This patch is an attempt to fix the ugly issue during migration:
  Cannot resync disks on node …: [True, 100]

If my understanding is correct, sometimes we poll the /proc/drbd file at
an inoportune moment, while it's being updated, or while the DRBD device
is changing state, and we see an unexpected state.

Based on the assumption that this is just a transient state, rather than
aborting directly, we change the backend.DrbdWaitSync() function to
retry a few times the operation, giving DRBD a chance to settle down at
the end of the resync.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>