KVM: VMX: Avoid exit when setting cr8 if the local apic is in the kernel

With apic in userspace, we must exit to userspace after a cr8 write in order
to update the tpr.  But if the apic is in the kernel, the exit is unnecessary.

Noticed by Joerg Roedel.

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