[libvirt] avoid "make rpm" failure in doc/

[Jim Meyering]

Eric Blake wrote:
> According to Jim Meyering on 2/25/2010 6:27 AM:
>> * docs/Makefile.am (libvirt-api.xml libvirt-refs.xml): Generalize
>> apibuild.py to work in a non-srcdir build.  Pass "srcdir" to it.
>> * docs/apibuild.py (rebuild): Honor the $srcdir envvar.
>
> ACK.
...

Thanks for the reviews.