Add a helper type and function for the mirror type

Currently we track the mirroring type as simply yes/no, with "yes"
meaning DRBD. This is not enough for handling shared storage, so we
need to add a type that gives the exact mirroring type (none,
internal, external), and a function that ties the disk template type
to the mirroring type.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>