KVM: VMX: Bring the common code to header file

This patch brings some of the code from vmx to x86.h header file. Now, we
can share this code between vmx and svm. Modified couple functions to make
it common.

Signed-off-by: Babu Moger <babu.moger@amd.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>