stand/common/disk.c: Read partition table relative to the start of the disk

If a disk is of an oddball size, like the 200mb + 512b used in rootgen.sh,
when disk_open() is called on a GELI encrypted partition, attempts to read
the partition table fail, as they pass through the decryption process which
turns the already plaintext data into jibberish.

When reading the partition table, always pass a slice and partition setting
of -1, and an offset of 0. Setting the slice to -1 prevents a false
positive when checking the slice against the cache of GELI encrypted
slices.

Reviewed by: imp, ian
Sponsored by: Klara Systems
Differential Revision: https://reviews.freebsd.org/D15847