x86/hvm: accelerate I/O intercept handling

x86/hvm: accelerate I/O intercept handling

currently we go through the emulator every time a HVM guest does an
I/O port access (in/out). This is unnecessary most of the times, as
both VMX and SVM provide all the necessary information already in the
VMCS/VMCB.  String instructions are not covered by this shortcut, but
they are quite rare and we would need to access the guest memory
anyway.  This patch decodes the information from VMCB/VMCS and calls a
simple handle_mmio wrapper. In handle_mmio() itself the emulation part
will simply be skipped, this approach avoids code duplication.  Since
the vendor specific part is quite trivial, I implemented both the VMX
and SVM part, please check the VMX part for sanity.

I boot-tested both versions and ran some simple benchmarks.  A micro
benchmark (hammering an I/O port in a tight loop) shows a significant
performance improvement (down to 66% of the time needed to handle the
intercept on an AMD K8, measured in the guest with TSC).  Even with
reading a 1GB file from an emulated IDE harddisk (Dom0 cached) I could
get a 4-5% improvement.  Some guest code (e.g. the TCP stack in some
Windows version) exercises the PM-Timer I/O port (0x1F48) very often
(multiple 10,000 times per second), these workloads also benefit with
up to 5% improvement from this patch.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>