Warn and fail if a device name is ambiguous.

The XI1 path bails out if the user specifies a device by name and there
is more than one device, but the XI2 path previously just silently chose
the first one. This patch makes it fail outright.

Signed-off-by: Will Thompson <will.thompson@collabora.co.uk>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>