blockdev: add x-blockdev-set-iothread testing command

Currently there is no easy way for iotests to ensure that a BDS is bound
to a particular IOThread.  Normally the virtio-blk device calls
blk_set_aio_context() when dataplane is enabled during guest driver
initialization.  This never happens in iotests since -machine
accel=qtest means there is no guest activity (including device driver
initialization).

This patch adds a QMP command to explicitly assign IOThreads in test
cases.  See qapi/block-core.json for a description of the command.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20171206144550.22295-9-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

qapi/block-core.json

@@ -3949,3 +3949,39 @@
   'data' : { 'parent': 'str',
              '*child': 'str',
              '*node': 'str' } }
+
+##
+# @x-blockdev-set-iothread:
+#
+# Move @node and its children into the @iothread.  If @iothread is null then
+# move @node and its children into the main loop.
+#
+# The node must not be attached to a BlockBackend.
+#
+# @node-name: the name of the block driver node
+#
+# @iothread: the name of the IOThread object or null for the main loop
+#
+# Note: this command is experimental and intended for test cases that need
+# control over IOThreads only.
+#
+# Since: 2.12
+#
+# Example:
+#
+# 1. Move a node into an IOThread
+# -> { "execute": "x-blockdev-set-iothread",
+#      "arguments": { "node-name": "disk1",
+#                     "iothread": "iothread0" } }
+# <- { "return": {} }
+#
+# 2. Move a node into the main loop
+# -> { "execute": "x-blockdev-set-iothread",
+#      "arguments": { "node-name": "disk1",
+#                     "iothread": null } }
+# <- { "return": {} }
+#
+##
+{ 'command': 'x-blockdev-set-iothread',
+  'data' : { 'node-name': 'str',
+             'iothread': 'StrOrNull' } }