loader: efipart should check disk size from partition table

While testing 32bit UEFI OVMF (which has bug about how the disk size
is presented), I did witness the errors from blkio->ReadBlocks().

It became apparent we can not entirely trust UEFI interfaces either,
so additional checks are needed.

So we use disk_ioctl(DIOCGMEDIASIZE) for disks, with fallback of
Media->LastBlock for other media.

In addition, we need to check if there is media present.

+ small fixes for error printout, and avoiding multiple blk * 512.

Reviewed by: allanjude
Approved by: allanjude (mentor)
Differential Revision: https://reviews.freebsd.org/D10197