resolve memory device roll over reporting issues with >32G guests

resolve memory device roll over reporting issues with >32G guests

The field within the Memory Device type 17 is only a word with the MSB being
used to report MB/KB.  Thereby, a guest with 32G and greater would report
incorrect memory device information rolling over to 0.

This presents more than one memory device and associated memory structures
if the memory is larger than 16G

This is port of commit e65bb0d2bd3a156408996674965555979de3a61b
from qemu pc-bios tree.

Signed-off-by: Gleb Natapov <gleb@redhat.com>