memory: allow destroying a non-empty MemoryRegion

memory: allow destroying a non-empty MemoryRegion

This is legal; the MemoryRegion will simply unreference all the
existing subregions and possibly bring them down with it as well.
However, it requires a bit of care to avoid an infinite loop.
Finalizing a memory region cannot trigger an address space update,
but memory_region_del_subregion errs on the side of caution and
might trigger a spurious update: avoid that by resetting mr->enabled
first.

Signed-off-by: Paolo Bonzini <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Message-Id: <1443689999[email protected]>