exec: allow to get a pointer for some mmio memory region

exec: allow to get a pointer for some mmio memory region

This introduces a special callback which allows to run code from some MMIO
devices.

SysBusDevice with a MemoryRegion which implements the request_ptr callback will
be notified when the guest try to execute code from their offset. Then it will
be able to eg: pre-load some code from an SPI device or ask a pointer from an
external simulator, etc..

When the pointer or the data in it are no longer valid the device has to
invalidate it.

Reviewed-by: Edgar E. Iglesias <[email protected]>
Signed-off-by: KONRAD Frederic <[email protected]>
Signed-off-by: Edgar E. Iglesias <[email protected]>