nic: do not destroy memory regions in cleanup functions

nic: do not destroy memory regions in cleanup functions

The memory regions should be destroyed in the unrealize function;
since these NICs are not even qdev-ified, they cannot be unplugged
and they do not have to do anything to destroy their memory regions.

Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Peter Crosthwaite <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>