Adjust the tcg_out_qemu_{ld,st}() slow paths to pass AREG0 in r3,
based on patches by malc.

Also adjust the registers clobbered, based on patch by Alex.
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Acked-by: NAlexander Graf <agraf@suse.de>
[AF: Do not hardcode r3 for AREG0, requested by Alex]
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>