test-qga: Actually test 0xff sync bytes

Commit 62c39b3 introduced test-qga, and at face value, appears
to be testing the 'guest-sync' behavior that is recommended for
guests in sending 0xff to QGA to force the parser to reset.  But
this aspect of the test has never actually done anything: the
qmp_fd() call chain converts its string argument into QObject,
then converts that QObject back to the actual string that is
sent over the wire - and the conversion process silently drops
the 0xff byte from the string sent to QGA, thus never resetting
the QGA parser.

An upcoming patch will get rid of the wasteful round trip
through QObject, at which point the string in test-qga will be
directly sent over the wire.

But fixing qmp_fd() to actually send 0xff over the wire is not
all we have to do - the actual QMP parser loudly complains that
0xff is not valid JSON, and sends an error message _prior_ to
actually parsing the 'guest-sync' or 'guest-sync-delimited'
command.  With 'guest-sync', we cannot easily tell if this error
message is a result of our command - which is WHY we invented
the 'guest-sync-delimited' command.  So for the testsuite, fix
things to only check 0xff behavior on 'guest-sync-delimited',
and to loop until we've consumed all garbage prior to the
requested delimiter, which is compatible with the documented actions
that a real QGA client is supposed to do.

Ideally, we'd fix the QGA JSON parser to silently ignore 0xff
rather than sending an error message back, at which point we
could enhance this test for 'guest-sync' as well as for
'guest-sync-delimited'.  But for the sake of this patch, our
testing of 'guest-sync' is no worse than it was pre-patch,
because we have never been sending 0xff over the wire in the
first place.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20170427215821.19397-11-eblake@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
[Additional comment squashed in, along with matching commit message
update]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
