[HVM] Use AMD's isntruction-length decoder for VMX as well as SVM

[HVM] Use AMD's isntruction-length decoder for VMX as well as SVM
MMIO decode. The VMX-defined instruction-length info field is not
valid for use during most page faults. Hence we have to obtain the
instruction length the slow-and-stupid way.

This *will* go away when we throw away the wretched MMIO emulator.

Signed-off-by: Keir Fraser <keir@xensource.com>