Only call stat() when adding a disk if we expect a device to exist.

We consider calling stat() a helpful error check in the following
circumstances only:
 1. the disk backend type must be PHYsical
 2. the disk backend domain must be the same as the running libxl
    code (ie LIBXL_TOOLSTACK_DOMID)
 3. there must not be a hotplug script because this would imply that
    the device won't be created until after the hotplug script has
    run.

With this fix, it is possible to use qemu's built-in block drivers
such as ceph/rbd, with a xl config disk spec like this:

disk=[ 'backendtype=qdisk,format=raw,vdev=hda,access=rw,target=rbd:rbd/ubuntu1204.img' ]

Signed-off-by: David Scott <dave.scott@eu.citrix.com>