kernel: Add more detailed VM detection.

kernel: Add more detailed VM detection.

Previously, the kernel global 'vmm_guest' was either 0 or 1 depending
on the VMM bit in the CPU features (which isn't set in all VMs).

This commit adds more detailed information by checking the emulated
BIOS for known strings. The detected VMs include vkernel, which
doesn't strictly fit into the category, but it shares enough
similarities for this to be useful.

Also expose this information in a read-only sysctl (kern.vmm_guest).

The detection code was kind of adapted from FreeBSD (although their
kern.vm_guest works differently).

Tested-by: tuxillo