Revision 326089

loader.efi: efipart does not recognize partitionless disks

Rework the block device handle check to allow more robust device
classification. This is mostly usability issue - it can be quite confusing
for user when no disks are listed with lsdev.

Add more comments about what and why is done.

Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D13026