target/ppc: Fix QEMU crash with stxsdx

target/ppc: Fix QEMU crash with stxsdx

I've been hitting several QEMU crashes while running a fedora29 ppc64le
guest under TCG. Each time, this would occur several minutes after the
guest reached login:

Fedora 29 (Twenty Nine)
Kernel 4.20.6-200.fc29.ppc64le on an ppc64le (hvc0)

Web console: https://localhost:9090/

localhost login:
tcg/tcg.c:3211: tcg fatal error

This happens because a bug crept up in the gen_stxsdx() helper when it
was converted to use VSR register accessors by commit 8b3b2d75c7c04
"target/ppc: introduce get_cpu_vsr{l,h}() and set_cpu_vsr{l,h}() helpers
for VSR register access".

The code creates a temporary, passes it directly to gen_qemu_st64_i64()
and then to set_cpu_vrsh()... which looks like this was mistakenly
coded as a load instead of a store.

Reverse the logic: read the VSR to the temporary first and then store
it to memory.

Fixes: 8b3b2d75c7c0481544e277dad226223245e058eb
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <155371035249.2038502.12364252604337688538.stgit@bahia.lan>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>