New APIs for context types and exportable resources
This is the first part of !412 (merged).
It adds virgl_renderer_resource_export_blob to export an fd for a blob resource. It also adds virgl_renderer_context_create_with_type to create a context with a specified type. There is currently only VIRGL_CTX_TYPE_VIRGL, but VIRGL_CTX_TYPE_VULKAN is planned.
There are also some minor additions to virgl_hw.h.