ANDROID: virtio_balloon: New module parameter "pkvm"

This specifies that the driver is running on a PKVM hypervisor
and must use the memrelinquish service to cooperatively release
memory. If this service is unavailable, virtio_balloon cannot be
used.

Bug: 240239989
Change-Id: I8800c4435d8fae9df6f1ab108cc61c8f93020773
Signed-off-by: Keir Fraser <keirf@google.com>