libvirt: volume snapshot delete for network-attached disks

Add support for performing a volume snapshot delete for
a network/(libgfapi)-attached disk based on the proposal
at:

http://www.redhat.com/archives/libvir-list/2014-February/msg01226.html

Uses a new VIR_DOMAIN_BLOCK_COMMIT_RELATIVE flag for blockCommit
calls which keeps qcow2 backing file entries as relative filenames
rather than prepending them with path information when blockCommit
operations are performed.

Support for this flag is required and heuristically detected rather
than based on version number.

Change-Id: I31710b787c39d23870fb45a460f460663ecb261c

Use vda[2] instead of vda:2 (squash)

WIP: use disk[1] references for base and top

Further changes

use index (squash)

Use libvirt flags for commit/rebase

Pass base=None for blockRebase

Fix up blockcommit relative detection