cfi02 is annoying in that is ignores some address bits; we probably
want explicit support in the memory API for that.

In order to get the correct opaque into the MemoryRegion object, the
allocation scheme is changed so that the flash emulation code allocates
memory, instead of the caller.  This clears a FIXME in the flash code.
Signed-off-by: NAvi Kivity <avi@redhat.com>