qemu: process: Refactor setup of memory ballooning

Since the monitor code now supports ullongs when setting balloon size,
drop the legacy code with overflow checking.

Additionally the comment mentioning that the job is treated as a sync
job does not make sense any more since the monitor is entered
asynchronously.

src/qemu/qemu_process.c

@@ -4270,8 +4270,6 @@ int qemuProcessStart(virConnectPtr conn,
     qemuDomainObjPrivatePtr priv = vm->privateData;
     virCommandPtr cmd = NULL;
     struct qemuProcessHookData hookData;
-    unsigned long cur_balloon;
-    int period = 0;
     size_t i;
     bool rawio_set = false;
     char *nodeset = NULL;
@@ -4880,28 +4878,24 @@ int qemuProcessStart(virConnectPtr conn,
     if (qemuDomainUpdateMemoryDeviceInfo(driver, vm, asyncJob) < 0)
         goto cleanup;
 
-    /* Technically, qemuProcessStart can be called from inside
-     * QEMU_ASYNC_JOB_MIGRATION_IN, but we are okay treating this like
-     * a sync job since no other job can call into the domain until
-     * migration completes.  */
     VIR_DEBUG("Setting initial memory amount");
-    cur_balloon = vm->def->mem.cur_balloon;
-    if (cur_balloon != vm->def->mem.cur_balloon) {
-        virReportError(VIR_ERR_OVERFLOW,
-                       _("unable to set balloon to %lld"),
-                       vm->def->mem.cur_balloon);
-        goto cleanup;
+    if (vm->def->memballoon &&
+        vm->def->memballoon->model != VIR_DOMAIN_MEMBALLOON_MODEL_NONE) {
+        unsigned long long balloon = vm->def->mem.cur_balloon;
+        int period = vm->def->memballoon->period;
+
+        if (qemuDomainObjEnterMonitorAsync(driver, vm, asyncJob) < 0)
+            goto cleanup;
+
+        if (period)
+            qemuMonitorSetMemoryStatsPeriod(priv->mon, period);
+
+        if (qemuMonitorSetBalloon(priv->mon, balloon) < 0)
+            goto exit_monitor;
+
+        if (qemuDomainObjExitMonitor(driver, vm) < 0)
+            goto cleanup;
     }
-    if (vm->def->memballoon && vm->def->memballoon->period)
-        period = vm->def->memballoon->period;
-    if (qemuDomainObjEnterMonitorAsync(driver, vm, asyncJob) < 0)
-        goto cleanup;
-    if (period)
-        qemuMonitorSetMemoryStatsPeriod(priv->mon, period);
-    if (qemuMonitorSetBalloon(priv->mon, cur_balloon) < 0)
-        goto exit_monitor;
-    if (qemuDomainObjExitMonitor(driver, vm) < 0)
-        goto cleanup;
 
     VIR_DEBUG("Detecting actual memory size for video device");
     if (qemuProcessUpdateVideoRamSize(driver, vm, asyncJob) < 0)