Improve partition detection on lesser used devices

The format strings in efi_get_info() are intended to extract both the
main device and partition number. However, this is only done correctly
for hd, sd and vd devices. The format strings for ram, dm-, md and loop
devices misparse the input. This causes the partition device to be
incorrectly labelled as the main device with the partition being
labelled 0.

Reported-by: ilovezfs <ilovezfs@icloud.com>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2175