blockdev: make image streaming safe across hotplug

Unplugging a storage interface like virtio-blk causes the host block
device to be deleted too.  Long-running operations like block migration
must take a DriveInfo reference to prevent the BlockDriverState from
being freed.  For image streaming we can do the same thing.

Note that it is not possible to acquire/release the drive reference in
block.c where the block job functions live because
drive_get_ref()/drive_put_ref() are blockdev.c functions.  Calling them
from block.c would be a layering violation - tools like qemu-img don't
even link against blockdev.c.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
