New API: is_whole_device
Fix bogus partition number passed to guestfs___check_for_filesystem_on

Partition number was being passed to guestfs___check_for_filesystem_on
based on an index in list_partition. However, this ignores the
possibility of multiple block devices.

This change makes guestfs___check_for_filesystem_on examine the
passed-in device directly to determine if it is a whole device, or
what its partition number is.
inspect: Don't re-implement list_filesystems in inspect_os