exec: introduce MemoryRegionCache

exec: introduce MemoryRegionCache

Device models often have to perform multiple access to a single
memory region that is known in advance, but would to use "DMA-style"
functions instead of address_space_map/unmap.  This can happen
for example when the data has to undergo endianness conversion.
Introduce a new data structure to cache the result of
address_space_translate without forcing usage of a host address
like address_space_map does.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>