anv: Teach generator scripts how to parse mutliple XML files

anv: Teach generator scripts how to parse mutliple XML files

The taught scripts are anv_extensions.py and anv_entrypoints_gen.py.  To
give a script multiple XML files, call it like so:

    anv_extensions.py --xml a.xml --xml b.xml --xml c.xml ...

The scripts parse the XML files in the given order.

This will allow us to feed the scripts XML files for extensions that are
missing from the official vk.xml, such as VK_ANDROID_native_buffer.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>