qmp-dispatch: Use CommandNotFound error for disabled commands

qmp-dispatch: Use CommandNotFound error for disabled commands

If a command is disabled an error is reported.  But due to usage of
error_setg() the class of the error is GenericError which does not
help callers in distinguishing this case from a case where a qmp
command fails regularly due to other reasons.

We used to use class CommandDisabled until the great error
simplification (commit de253f1491 for QMP and commit 93b91c59db for
qemu-ga, both v1.2.0).

Use CommandNotFound error class, which is close enough.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Message-Id: <faeb030e6a1044f0fd88208edfdb1c5fafe5def9.1567171655.git.mprivozn@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[Test update squashed in, commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>