spapr: Convert hpt_prepare_thread() to use qemu_try_memalign()

HPT resizing is asynchronous: the guest first kicks off the creation of a
new HPT, then it waits for that new HPT to be actually created and finally
it asks the current HPT to be replaced by the new one.

In the case of a userland allocated HPT, this currently relies on calling
qemu_memalign() which aborts on OOM and never returns NULL. Since we seem
to have path to report the failure to the guest with an H_NO_MEM return
value, use qemu_try_memalign() instead of qemu_memalign().

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <160398563636.32380.1747166034877173994.stgit@bahia.lan>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>