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>