CP-6172: Support configurable VBD backend path in Xenstore

This allows Blktap2 and Blktap3 to coexist.

Note: the backend path is specified by a new "backend-kind" key in the
extra_backend_keys of the VBD metadata.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Re-enable build of watch_test binary
Added timeout testing to watch_test (FIXME)

The watch_test is not run as part of the build since it needs to be run in
dom0. Somewhere along the line, watches became broken in that they do not
timeout after the specified time.