Add VBD.other_config:backend-kind for blktap3

Note: this change is co-dependent on a change in xenopsd.

Blktap3 and kernel blkback need to co-exist and, since kernel blkback watches
keys in the /local/domain/0/vbd/ tree, the paths for blktap3 need to be moved.

A new key has been added to the other_config field of the VBD record which is
passed through to xenopsd to determine where to place the VBD's XenStore
entries.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>