tests: utils: poll fillmem for memory dirtying completion

`make_guest_dirty_memory` previously ran fillmem via SSH with a one
second timeout. If fillmem took longer than 1 second, or if the SSH
connection was killed by the OOM killer, the framework would report an
error, and we have no idea if the correct amount of memory was dirtied.

To avoid this, start fillmem detached using `nohup` and poll the status
file that it writes until it reports success. test_hotplug_memory.py
already checks that status file after running fillmem; a later commit
switches it to this helper. In some of the balloon tests, we expect the
OOM killer to kill fillmem, in which case the status file would never be
written — we add an `oom_expected` escape hatch for this case, which
just sleeps for 5 seconds (matching the old behaviour).

Signed-off-by: James Curtis <jxcurtis@amazon.co.uk>