Remove VHD backend for now

This needs to be refactored given the big interface changes in the
vhd library.
Update to new xenstore interface
Release 1.1.0

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge branch 'master' of git://github.com/mirage/xen-disk

Conflicts:
	CHANGES
Merge pull request #4 from djs55/master

Fix build against more recent libraries