curl: Don't deref NULL pointer in call to aio_poll.

In commit 63f0f45f the following
mechanical change was made:

         if (!state) {
-            qemu_aio_wait();
+            aio_poll(state->s->aio_context, true);
         }

The new code now checks if state is NULL and then dereferences it
('state->s') which is obviously incorrect.

This commit replaces state->s->aio_context with
bdrv_get_aio_context(bs), fixing this problem.  The two other hunks
are concerned with getting the BlockDriverState pointer bs to where it
is needed.

The original bug causes a segfault when using libguestfs to access a
VMware vCenter Server and doing any kind of complex read-heavy
operations.  With this commit the segfault goes away.
Signed-off-by: NRichard W.M. Jones <rjones@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NBenoît Canet <benoit.canet@nodalink.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/curl.c

@@ -357,7 +357,7 @@ static void curl_multi_timeout_do(void *arg)
 #endif
 }
 
-static CURLState *curl_init_state(BDRVCURLState *s)
+static CURLState *curl_init_state(BlockDriverState *bs, BDRVCURLState *s)
 {
     CURLState *state = NULL;
     int i, j;
@@ -375,7 +375,7 @@ static CURLState *curl_init_state(BDRVCURLState *s)
             break;
         }
         if (!state) {
-            aio_poll(state->s->aio_context, true);
+            aio_poll(bdrv_get_aio_context(bs), true);
         }
     } while(!state);
 
@@ -566,7 +566,7 @@ static int curl_open(BlockDriverState *bs, QDict *options, int flags,
     DPRINTF("CURL: Opening %s\n", file);
     s->aio_context = bdrv_get_aio_context(bs);
     s->url = g_strdup(file);
-    state = curl_init_state(s);
+    state = curl_init_state(bs, s);
     if (!state)
         goto out_noclean;
 
@@ -651,7 +651,7 @@ static void curl_readv_bh_cb(void *p)
     }
 
     // No cache found, so let's start a new request
-    state = curl_init_state(s);
+    state = curl_init_state(acb->common.bs, s);
     if (!state) {
         acb->common.cb(acb->common.opaque, -EIO);
         qemu_aio_release(acb);