vulkan: provide vk.xml as argument to the python generator

vulkan: provide vk.xml as argument to the python generator

Do not hardcode the file in the python script, but pass it via the build
system(s). The latter is the only one that should know about the file
location/tree structure.

Cc: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>