Change dm::list_devices to return a Device instead of u64
Change dm::list_devices to return a Device instead of u64

A Device can easily be converted to major:minor or the path, so that's
handy.

Signed-off-by: Andy Grover <b7d524d2f5cc5aebadb6b92b08d3ab26911cde33@redhat.com>

agrover/melvin