rbd: Add support for wiping RBD volumes using TRIM.

rbd: Add support for wiping RBD volumes using TRIM.

Using VIR_STORAGE_VOL_WIPE_ALG_TRIM a RBD volume can be trimmed down
to 0 bytes using rbd_discard()

Effectively all the data on the volume will be lost/gone, but the volume
remains available for use afterwards.

Starting at offset 0 the storage pool will call rbd_discard() in stripe
size * count increments which is usually 4MB. Stripe size being 4MB and
count 1.

rbd_discard() is available since Ceph version Dumpling (0.67) which dates
back to August 2013.

Signed-off-by: Wido den Hollander <wido@widodh.nl>