KVM: move complete_insn_gp() into x86.c

move the complete_insn_gp() helper function out of the VMX part
into the generic x86 part to make it usable by SVM.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>