unmodified_drivers: unplug the emulated devices at resume time

unmodified_drivers: unplug the emulated devices at resume time

Since qemu-2.10 it is required to unplug emulated devices again after
a live migration. If this is not done, qemu's block-backend driver
will be unable to open the backing disk image because it is still busy
by qemu's IDE driver. As a result the domUs block-frontend driver will
be unable to access the disks, and the domU has to be destroyed.
libxl is unable to detect the situation.

Apply the same workaround for this qemu bug that was done already
years ago in linux.git with commit 512b109ec962 ("xen: unplug the
emulated devices at resume time") to make sure xenlinux based domUs
can be migrated to unfixed hosts.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Jan Beulich <jbeulich@suse.com>