KVM: Add mmu cache clear function

Functions that play around with the physical memory map
need a way to clear mappings to possibly nonexistent or
invalid memory.  Both the mmu cache and the processor tlb
are cleared.

Signed-off-by: Dor Laor <dor.laor@qumranet.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>