sys/arch/amd64: support ramdisks when booting in GENPVH mode

This adds support for booting NetBSD/amd64 with QEMU’s -kernel/-initrd
combination, in generic PVH mode (GENPVH). This includes the MICROVM
kernel, which now defaults to supporting modules and root on memory
disks.

This patch probes the first few bytes loaded in memory for
differentiating between modules, splash screens (PNG or JPEG), or
assumes filesystem images otherwise. It works with plain filesystem
images, but not with modules yet. Supporting any combination of
filesystem, modules or splash screen images should be possible in the
future.

Tested on NetBSD/amd64 by iMil and myself; measured to gain another 1~3
ms of boot time with the MICROVM kernel on the reference hardware.

As posted on tech-kern@ on 26/11/2025, with no objections.

XXX pull-up to -11 and adapt for -10

Add `+invtsc` to the QEMU command line example, this enables the
invariant TSC feature, allowing the kernel to use CPUID 0x40000010 to
read TSC and LAPIC frequencies from the host, thus avoiding the need
for frequency calculation and gaining over 200ms in boot speed.

branches: 1.3.2; 1.3.4;
x86: consolidate MICROVM kernel configurations

Move common configuration options from amd64/conf/MICROVM and
i386/conf/MICROVM into a shared x86/conf/MICROVM.common file.

virtio(4): Consolidate kernel configs.

No functional change intended.

Leave `# XXX ?' comments where I don't know why the driver in
question is excluded. (Typical reason is that PAGE_SIZE is not 4096
but I didn't investigate -- if you do investigate, please either
update the comment if you determine a reason, or enable the driver if
you don't.)

PR kern/59211: vio9p(4): missing from various GENERICs and MAKEDEVs

Add support for command line MMIO devices, at least qemu and
Firecracker pass MMIO virtual devices mapping through the kernel
command line.
This driver is based on Colin Percival's FreeBSD virtio_mmio_cmdline.c
https://github.com/freebsd/freebsd-src/blob/main/sys/dev/virtio/mmio/virtio_mmio_cmdline.c

The following kernel options are needed

options MPBIOS
options MPTABLE_LINUX_BUG_COMPAT

As are these drivers

pv* at pvbus?
virtio* at pv?

Exemple qemu usage on a Linux host to boot a NetBSD guest:

qemu-system-x86_64 \
-M microvm,x-option-roms=off,rtc=on,acpi=off,pic=off,accel=kvm \
-m 256 -cpu host -kernel ${KERNEL} \
-append "root=ld0a console=com rw -v" \
-device virtio-blk-device,drive=hd0 \
-drive file=${IMG},format=raw,id=hd0 \
-device virtio-net-device,netdev=net0 \
-netdev user,id=net0,ipv6=off,hostfwd=::2200-:22 \
-global virtio-mmio.force-legacy=false -display none -serial stdio

A lightweight kernel configuration named MICROVM if available for this
use case.