vfio/migration: Correct device state from vmstate change for savevm case

vfio/migration: Correct device state from vmstate change for savevm case

Set _SAVING flag for device state from vmstate change handler when it
gets called from savevm.

Currently State transition savevm/suspend is seen as:
    _RUNNING -> _STOP -> Stop-and-copy -> _STOP

State transition savevm/suspend should be:
    _RUNNING -> Stop-and-copy -> _STOP

State transition from _RUNNING to _STOP occurs from
vfio_vmstate_change() where when vmstate changes from running to
!running, _RUNNING flag is reset but at the same time when
vfio_vmstate_change() is called for RUN_STATE_SAVE_VM, _SAVING bit
should be set.

Reported by: Yishai Hadas <yishaih@nvidia.com>
Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
Message-Id: <1623177441-27496-1-git-send-email-kwankhede@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>