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 non-Xen PVH guests to i386, PR kern/57813
Tested on QEMU by me, Xen PV & PVH by bouyer@.