test-qga: Clean up how we test QGA synchronization

test-qga: Clean up how we test QGA synchronization

To permit recovering from arbitrary JSON parse errors, the JSON parser
resets itself on lexical errors.  We recommend sending a 0xff byte for
that purpose, and test-qga covers this usage since commit 5229564b832.
That commit had to add an ugly hack to qmp_fd_vsend() to make capable
of sending this byte (it's designed to send only valid JSON).

The previous commit added a way to send arbitrary text.  Put that to
use for this purpose, and drop the hack from qmp_fd_vsend().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180823164025.12553-8-armbru@redhat.com>