qemu: Add coverity[negative_returns] tag

This avoids "Event negative_returns: A negative constant "-1" is passed as
an argument to a parameter that cannot be negative.".  The called function
uses -1 to determine whether it needs to traverse all the hostdevs.

src/qemu/qemu_hotplug.c

@@ -2539,6 +2539,7 @@ qemuDomainDetachNetDevice(virQEMUDriverPtr driver,
     detach = vm->def->nets[detachidx];
 
     if (virDomainNetGetActualType(detach) == VIR_DOMAIN_NET_TYPE_HOSTDEV) {
+        /* coverity[negative_returns] */
         ret = qemuDomainDetachThisHostDevice(driver, vm,
                                              virDomainNetGetActualHostdev(detach),
                                              -1);