iotests: Test resizing image attached to an iothread

iotests: Test resizing image attached to an iothread

This tests that trying to resize an image with QMP block_resize doesn't
hang or otherwise fail when the image is attached to a device running in
an iothread.

This is a regression test for the recent fix that changed
qmp_block_resize, which is a coroutine based QMP handler, to avoid
calling no_coroutine_fns directly.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230509134133.373408-1-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>