call HotplugHandler->plug() as the last step in device realization

call HotplugHandler->plug() as the last step in device realization

When [2] was fixed it was agreed that adding and calling post_plug()
callback after device_reset() was low risk approach to hotfix issue
right before release. So it was merged instead of moving already
existing plug() callback after device_reset() is called which would
be more risky and require all plug() callbacks audit.

Looking at the current plug() callbacks, it doesn't seem that moving
plug() callback after device_reset() is breaking anything, so here
goes agreed upon [3] proper fix which essentially reverts [1][2]
and moves plug() callback after device_reset().
This way devices always comes to plug() stage, after it's been fully
initialized (including being reset), which fixes race condition [2]
without need for an extra post_plug() callback.

 1. (25e897881 "qdev: add HotplugHandler->post_plug() callback")
 2. (8449bcf94 "virtio-scsi: fix hotplug ->reset() vs event race")
 3. https://www.mail-archive.com/qemu-devel@nongnu.org/msg549915.html

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1539696820-273275-1-git-send-email-imammedo@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-by: Pierre Morel<pmorel@linux.ibm.com>
Acked-by: Pierre Morel<pmorel@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>