Install API XML desc to a standard location

Install API XML desc to a standard location

Language bindings may well want to use the libvirt-api.xml and
libvirt-qemu-api.xml files to either auto-generate themselves,
or sanity check the manually written bindings for completeness.
Currently these XML files are not installed as standard, merely
ending up as a %doc file in the RPM.

This changes them to be installed into $prefix/share/libvirt/apis/
The *-refs.xml files are not installed, since those are only
useful during generation of the online API doc files.

The pkg-config file is enhanced so that you can query the install
location of the API files. eg

  # pkg-config --variable=libvirt_qemu_api libvirt
  /home/berrange/builder/i686-pc-mingw32/sys-root/mingw/share/libvirt/libvirt-qemu-api.xml

* docs/Makefile.am: Install libvirt-api.xml & libvirt-qemu-api.xml
* libvirt.pc.in: Add vars for querying API install location
* libvirt.spec.in, mingw32-libvirt.spec.in: Include API XML files