xgl-generate.py: rewrite LoaderEntrypointsSubcommand
xgl-generate.py: rewrite LoaderEntrypointsSubcommand

With the goal to generalize dispatch rules and avoid code duplication.  As a
result, xglOpenPeerImage() is fixed and xglAllocDescriptorSets() gains error
checking.

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools