virtio_balloon: stay awake while adjusting balloon

A virtio_balloon's parent device may be configured so that a
configuration change interrupt is a wakeup event. Extend the processing
of such a wakeup event until the balloon finishes inflating or deflating
by calling pm_stay_awake/pm_relax in the virtio_balloon driver. Note
that these calls are no-ops if the parent device doesn't support wakeup
events or if the wakeup events are not enabled.

This change allows the guest to use system power states such as s2idle
without running the risk the virtio_balloon's cooperative memory
management becoming unresponsive to the host's requests.

Tested-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: David Stevens <stevensd@chromium.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Message-Id: <20240110021925.1137333-1-stevensd@google.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>