The attached patch is a hack I cooked up to fix one of the libvirt-TCK
Xen failures.  The test (200-disk-hotplug.t) attempts to hot add and
remove a disk from a running domain.  The add works fine, but remove
fails with

libxl: debug: libxl.c:3858:libxl_device_disk_remove: ao 0x7f9b9c0015a0:
create: how=(nil) callback=(nil) poller=
0x7f9ba0004590
libxl: error: libxl.c:2399:libxl__device_from_disk: unrecognized disk
backend type: 0

The test does not define a backend type, in which case the libvirt libxl
driver allows libxl to choose an appropriate backend via
libxl__device_disk_set_backend().  The backend type is never set on a
remove operation, hence it fails with the above error.

I spent some time trying to figure out the best place to set backend
type on remove, but in the end could only come up with this hack.  It
wouldn't feel so gross if I could have simply added
'libxl__device_##type##_setdefault(gc, type);' to the existing
DEFINE_DEVICE_REMOVE macro, but alas libxl__device_nic_setdefault() has
a different prototype than the other devices.

Better suggestions welcomed!  One I considered was fixing this in
libvirt.  But the Xen community suggested allowing libxl to choose a
suitable backend when not specified, so I think this recommendation
should be symmetrical in the add and remove operations.

Regards,
Jim