KVM: VMX: Add facility to atomically switch MSRs on guest entry/exit

Some guest msr values cannot be used on the host (for example. EFER.NX=0),
so we need to switch them atomically during guest entry or exit.

Add a facility to program the vmx msr autoload registers accordingly.

Signed-off-by: Avi Kivity <avi@redhat.com>