[media] mem2mem_testdev: rename to vim2m

This is 1) *much* easier to type, and 2) is consistent with vivid
('vi' for virtual). More of such virtual drivers are planned, so keeping
the naming consistent makes sense.

Note that the old module name is retained as a module alias.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>