Extended memory here is "physical memory above 256MB".
"memsize" in the environment only grows to 256MB; "ememsize" is the entire
memory range. Extended memory shows up at physical address 0x90000000.
This allows for malta64 VMs to be created with > 256MB RAM, all the way
up to 2GB RAM.
Tested:
* qemu-devel package; qemu-system-mips64 -m 2048 (and -m 256 to test the
no-ememsize case.)
TODO:
* testing mips32 with > 256MB RAM.
Reviewed by: imp
b5c03ef320