build-sys: add qapi doc generation targets

Generate and install the man, txt and html versions of QAPI
documentation (generate and install qemu-doc.txt too).

Add it also to optional pdf/info targets.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170113144135.5150-22-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>