nvmm: change the api

[Maxime Villard <max%m00nbsd.net@localhost>]

I intend to change the NVMM API in order to reduce the data movements. The
patches are available here [1].

Basically until now the API expected the user to create structures and then
pass them in a libnvmm function. Typically:

	/* The structures. */
	struct nvmm_machine mach;
	struct nvmm_x64_state state;
	struct nvmm_event event;
	struct nvmm_exit exit;

	/* Create the machine and VCPU0. */
	nvmm_machine_create(&mach);
	nvmm_vcpu_create(&mach, 0);

	/* Set RAX=123. */
	nvmm_vcpu_getstate(&mach, 0, &state, NVMM_X64_STATE_GPRS);
	state.gprs[NVMM_X64_GPR_RAX] = 123;
	nvmm_vcpu_setstate(&mach, 0, &state, NVMM_X64_STATE_GPRS);

	/* Inject an #UD exception. */
	event.type = NVMM_EXCEPTION;
	event.vector = 6;
	nvmm_vcpu_inject(&mach, 0, &event);

	/* Run. */
	nvmm_vcpu_run(&mach, 0, &exit);
	switch (exit.reason) {
		...
	}

This design has some problems: the structures can occupy a lot of stack
(nvmm_x64_state is > 1000 bytes), and must be copied in and out of the
comm page by libnvmm. We can avoid this by giving the user direct access
to the comm page.

A new nvmm_vcpu structure is introduced, and contains pointers to the comm
page. The new usage is:

	/* The structures. */
	struct nvmm_machine mach;
	struct nvmm_vcpu vcpu;

	/* Create the machine and VCPU0. */
	nvmm_machine_create(&mach);
	nvmm_vcpu_create(&mach, 0, &vcpu);

	/* Set RAX=123. getstate populates vcpu.state->gprs[]. */
	nvmm_vcpu_getstate(&mach, &vcpu, NVMM_X64_STATE_GPRS);
	vcpu.state->gprs[NVMM_X64_GPR_RAX] = 123;
	nvmm_vcpu_setstate(&mach, &vcpu, NVMM_X64_STATE_GPRS);

	/* Inject an #UD exception. */
	vcpu.event->type = NVMM_EXCEPTION;
	vcpu.event->vector = 6;
	nvmm_vcpu_inject(&mach, &vcpu);

	/* Run. */
	nvmm_vcpu_run(&mach, &vcpu);
	switch (vcpu.exit->reason) {
		...
	}

No kernel changes are required. This should be part of NetBSD 9.

[1] https://m00nbsd.net/garbage/nvmm/nvmm-nocopy.zip