docs/interop/qmp-spec: How to force known good parser state

docs/interop/qmp-spec: How to force known good parser state

Section "QGA Synchronization" specifies that sending "a raw 0xFF
sentinel byte" makes the server "reset its state and discard all
pending data prior to the sentinel."  What actually happens there is a
lexical error, which will produce one or more error responses.
Moreover, it's not specific to QGA.

Create new section "Forcing the JSON parser into known-good state" to
document the technique properly.  Rewrite section "QGA
Synchronization" to document just the other direction, i.e. command
guest-sync-delimited.

Section "Protocol Specification" mentions "synchronization bytes
(documented below)".  Delete that.

While there, fix it not to claim '"Server" is QEMU itself', but
'"Server" is either QEMU or the QEMU Guest Agent'.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180823164025.12553-2-armbru@redhat.com>