Rationalise lvm2 plug-in lvmdump options

Backport of commit 89ef36a.

The lvm2 lvmdump options were a bit crazy, allowing the user to
specify a default ('lvmdump'), or an advanced collection
('lvmdump-a') but not the raw metadata ('-m') option which is
often useful for support purposes.

Replace 'lvmdump-a' with 'lvmdump-am' to collect both.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>