storage: if LVM volume groups are available, allow an SR to be created on one

This uses the ezlvm script-based storage adapter. Note the storage will
use raw block devices and blkback -- no blkback dependency.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Start the xapi-storage-script service (needed by ezlvm)

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