boards: x86: increase VM size on PC-like

These are all PC systems which have large amounts of memory
which needs to be mapped at runtime (most are 2GB).

Increase the address space size accordingly, adding an extra
8MB for mappings.

The ACRN target has 8MB, give it 16MB of VM.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>