Fix a regression in "movs" emulation after r284539. The regression was caused
due to a change in behavior of the 'vm_map_gpa()'.

Prior to r284539 if 'vm_map_gpa()' was called to map an address range in the
guest MMIO region then it would return NULL. This was used by the "movs"
emulation to detect if the 'src' or 'dst' operand was in MMIO space.

Post r284539 'vm_map_gpa()' started returning a non-NULL pointer even when
mapping the guest MMIO region.

Fix this by returning non-NULL only if [gaddr, gaddr+len) is entirely
within the 'lowmem' or 'highmem' regions and NULL otherwise.

Pointy hat to: neel
Reviewed by: grehan
Reported by: tychon, Ben Perrault (ben.perrault@gmail.com)
MFC after: 1 week