vfio-helpers: Fix qemu_vfio_open_pci() crash

qemu_vfio_open_common() initializes s->lock only after passing s to
qemu_vfio_dma_map() via qemu_vfio_init_ramblock().
qemu_vfio_dma_map() tries to lock the uninitialized lock and crashes.

Fix by initializing s->lock first.

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1645840


Fixes: 418026ca
Cc: qemu-stable@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20181127084143.1113-1-armbru@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

util/vfio-helpers.c

@@ -411,13 +411,13 @@ static int qemu_vfio_init_ramblock(const char *block_name, void *host_addr,
 
 static void qemu_vfio_open_common(QEMUVFIOState *s)
 {
+    qemu_mutex_init(&s->lock);
     s->ram_notifier.ram_block_added = qemu_vfio_ram_block_added;
     s->ram_notifier.ram_block_removed = qemu_vfio_ram_block_removed;
     ram_block_notifier_add(&s->ram_notifier);
     s->low_water_mark = QEMU_VFIO_IOVA_MIN;
     s->high_water_mark = QEMU_VFIO_IOVA_MAX;
     qemu_ram_foreach_block(qemu_vfio_init_ramblock, s);
-    qemu_mutex_init(&s->lock);
 }
 
 /**