For non-RAM memory regions, we cannot tell whether this is an I/O region
or an MMIO region.  Since the qemu backing registration is different for
the two, we have to defer initialization until we know which address
space we are in.

These shenanigans will be removed once the backing registration is unified
with the memory API.
Signed-off-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>