target/arm/hvf: Mention hvf_sync_vtimer() must run on vCPU thread

Since hvf_sync_vtimer() calls hv_vcpu_get_sys_reg(),
which must run on a vCPU, it also must. Mention it.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Mads Ynddal <mads@ynddal.dk>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/arm/hvf/hvf.c

@@ -1764,6 +1764,7 @@ static void hvf_wfi(CPUState *cpu)
     hvf_wait_for_ipi(cpu, &ts);
 }
 
+/* Must be called by the owning thread */
 static void hvf_sync_vtimer(CPUState *cpu)
 {
     ARMCPU *arm_cpu = ARM_CPU(cpu);