connect: check if 'id' uniquely matches a backend 'params' key

From the user's point of view they can now write:
* target=/dev/loop0 in the xl/xm config file
* [connect "/dev/loop0"] will match this block device.

Normally we shouldn't rely on knowing about the backend's internal
structure (it could be a special driver domain), but for simple
cases (such as when developing locally on a laptop) it's convenient
to use the dom0 file or device name.

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