dataplane: refuse to start if device is already in use

dataplane: refuse to start if device is already in use

Dataplane must check whether a block device is in use before launching
the dataplane thread.  This is necessary since the thread does not
synchronize with the main loop and I/O requests could cause corruption.

One example is when a drive is added and a block job is started before
hotplugging the virtio-blk-pci adapter.  In this case we must not use
dataplane mode.

Cc: qemu-stable@nongnu.org
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit b0f2027cde31c645524256763672e09eeb204a9a)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>