KVM: Align cr8 threshold when userspace changes cr8

Commit f0a3602c20 ("KVM: Move interrupt injection logic to x86.c") does not
update the cr8 intercept if the lapic is disabled, so when userspace updates
cr8, the cr8 threshold control is not updated and we are left with illegal
control fields.

Fix by explicitly resetting the cr8 threshold.

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