lxd/instance/drivers/driver/qemu: Updates onStop to be more like lxc driver
lxd/instance/drivers/driver/qemu: Updates onStop to be more like lxc driver

 - Use defer to cancel operation lock.
 - Move shutdown event to be sent at same point as lxc container.
 - Record last power state before device cleanup (sameas lxc container).

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

hallyn/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd