KVM: x86: preserve the high 32-bits of the PAT register

KVM does not really do much with the PAT, so this went unnoticed for a
long time.  It is exposed however if you try to do rdmsr on the PAT
register.
Reported-by: NValentine Sinitsyn <valentine.sinitsyn@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -461,7 +461,7 @@ struct kvm_vcpu_arch {
 	bool nmi_injected;    /* Trying to inject an NMI this entry */
 
 	struct mtrr_state_type mtrr_state;
-	u32 pat;
+	u64 pat;
 
 	unsigned switch_db_regs;
 	unsigned long db[KVM_NR_DB_REGS];