Move the check of dmsetup to check_dm_target_support, and adapt the only
two places checking if dmsetup is present in the system. Now we skip the
test if dmsetup isn't available, instead of marking the test as failed.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>