nbd: add missed aio_context_acquire in nbd_export_new

blk_invalidate_cache() can call qcow2_invalidate_cache which performs
IO inside.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1453273940-15382-3-git-send-email-den@openvz.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

nbd/server.c

diff --git a/nbd/server.c b/nbd/server.c
index eead339..3596e68 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -671,7 +671,9 @@
      * that BDRV_O_INCOMING is cleared and the image is ready for write
      * access since the export could be available before migration handover.
      */
+    aio_context_acquire(exp->ctx);
     blk_invalidate_cache(blk, NULL);
+    aio_context_release(exp->ctx);
     return exp;
 
 fail: