lvmstrap: add support for non-partitioned md disks

This patch, originally written by Marc Schmitt <mschmitt@google.com>,
adds support for MD devices (used in a non-partitioned mode). I
abstracted all the original startswith('md') checks into separate
functions, and also moved the supported disk types to a list.

Proper "in-use" detection also needs another check, which will come in
a subsequent patch.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>