build-sys: Minor qapi doc generation target cleanups

Move makeinfo flags from MAKEINFO to MAKEINFOFLAGS.  Fix the call of
quiet-command for target qemu-ga-qapi.texi.  Both messed up in commit
56e8bdd4.

Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1484816804-12598-1-git-send-email-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>