vkr: add a few resource validations

Summary:

• validate shmem resource type before use
• protect reply encoder resource from invalid resource destroy
• simplify nested decoder resource and validate against invalid resource destroy

Stat:

• Closes: https://gitlab.freedesktop.org/virgl/virglrenderer/-/issues/497
• Closes: https://gitlab.freedesktop.org/virgl/virglrenderer/-/issues/499
• No perf regression in trace replays and vkmark

/cc @gerddie @tintou