partitions: fix empty parititon table entry detection

Partitions table entries with cyl=0 or head=0 are legal, avoid false
positives.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
partitions: remove bad checks on paritition table entries

head=254 is legal, and the check also assumes that heads=255, which is not
a given.  Remove the check and trust the LBA.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
partitions: fix partition offset truncation

The partition offset can be > 32 bits, so allow for it in the calculation.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Merge branch 'partitions'

Partition table fixes.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>