Fix bhyve SMBIOS type 19 handling to avoid misreporting total RAM amount

This fixes the amount of memory displayed in the EDK2 UiApp to be the same
as passed on the bhyve command line. Otherwise, 8GB is displayed as 4GB,
32GB as 28GB etc.

Reviewed by: jhb, kib, rgrimes
Differential Revision: https://reviews.freebsd.org/D27348