qemu-iotests: Allow QMP pretty printing in common.qemu

QMP responses to certain commands can become quite long, which doesn't
only make reading them hard, but also means that the maximum line length
in patch emails can be exceeded. Allow tests to switch to QMP pretty
printing, which results in more, but shorter lines.

We also need to make sure to keep indentation in the response for this
to work as expected.

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