KVM: i8254: tone down WARN_ON pit.state_lock

If the guest could hit this, it would hang the host kernel, bacause of
sheer number of those reports.  Internal callers have to be sensible
anyway, so we now only check for it in an API function.

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>