sysbus_init_mmio_cb() uses the destructive IO_MEM_UNASSIGNED to remove a
region.  Provide an alternative that calls an unmap callback, so the removal
may be done non-destructively.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>