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

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>