Hello.  I made some changes to /sys/src/9/^(pc pc64) and
/sys/src/cmd/vmx to get better support for Linux and OpenBSD emulation
and before I push it (or not) I'd like to get more eyes (and hands) on
it.  386 kernel should not be affected by the change at all, this is
about amd64 specifically.

Changes were tested on two machines I have, with AVX enabled/disabled.
No problems were found with extensive use, nor any performance issues
detected.

On the host Go is using AVX successfully with this change.  I'm
writing optimized routines related to video playback so that's yet
another reason why this work has been done in the first place.

* AVX/AVX2 support on amd64 for both 9front kernel itself + VMX
  guests.  Enabled by setting "*avx=" in plan9.ini.
* Make vmx(1) report to guest it's running under a hypervisor.
* Provide "fast strings" (through msr) properly to guests.
* Rework cpuid in vmx(1).
* A bit better timing by using tsc offset feature.  Clock is still
  wrong but at least not THAT much.  Proper kvm clock implementation
  in the future will address that.

Thanks.

 - Sigrid