test(vmm): verify unplugged memory stays protected

Add an integration test that plugs the whole virtio-mem region and
unplugs it, then checks the Firecracker process address space: the
plug/unplug cycle must not leave a new writable anonymous mapping, i.e.
unplugged slots must remain PROT_NONE after discard_range remaps them.

Runs across the uvm_any_memhp matrix, so it covers the file-backed
(snapshot-restored) discard path that the in-tree unit tests do not,
since those use anonymous guest memory (the madvise branch).

Signed-off-by: Jay Chung <jaehoc@amazon.com>