loader: zfs reader should not probe partitionless disks (UEFI case)

With r342151 I did fix the BIOS version of zfs_probe_dev() from accessing
the whole disk, but the fix was not complete - we actually did not check
if the device name was really for whole disk. Since UEFI version
is only calling the zfs_probe_dev() with partitions and not with whole
disk, the UEFI loader was not able to find the zfs pools.

This update does correct the issue by calling archsw.arch_getdev() to
translate the device name back to dev_desc, and we have whole disk when both
partition and slice values are -1.

Reported by: alvisen_gmail.com
Differential Revision: https://reviews.freebsd.org/D18558