KVM: VMX: Don't switch 64-bit msrs for 32-bit guests

Some msrs are only used by x86_64 instructions, and are therefore
not needed when the guest is legacy mode.  By not bothering to switch
them, we reduce vmexit latency by 2400 cycles (from about 8800) when
running a 32-bt guest on a 64-bit host.

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