Make virtual media use urlFromPieces

urlFromPieces is the new way to construct "correct" urls.  It also
cleans up quite a bit of this code.  Use it.

Tested: Code compiles.  No backend in upstream yet.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ic43c257613061708e96a5449e0b3d57f288fbffa