blkdeviotune: check for overflow when parsing XML

According to docs/schemas/domaincommon.rng and _virDomainBlockIoTuneInfo
all the iotune values are interpreted as unsigned long long, however
according to qemu_monitor_json.c, qemu silently truncates numbers
larger than LLONG_MAX. There's really not much of a usage for such
large numbers anyway yet. This patch provides the same overflow
check during a domain start as it does during setting
a blkdeviotune element in qemu_driver.c and thus reports an error when
a larger number than LLONG_MAX is detected.

https://bugzilla.redhat.com/show_bug.cgi?id=1131876

src/qemu/qemu_command.c

@@ -3620,6 +3620,18 @@ qemuBuildDriveStr(virConnectPtr conn,
         goto error;
     }
 
+    if (disk->blkdeviotune.total_bytes_sec > LLONG_MAX ||
+        disk->blkdeviotune.read_bytes_sec > LLONG_MAX ||
+        disk->blkdeviotune.write_bytes_sec > LLONG_MAX ||
+        disk->blkdeviotune.total_iops_sec > LLONG_MAX ||
+        disk->blkdeviotune.read_iops_sec > LLONG_MAX ||
+        disk->blkdeviotune.write_iops_sec > LLONG_MAX) {
+        virReportError(VIR_ERR_OVERFLOW,
+                      _("block I/O throttle limit must "
+                        "be less than %llu using QEMU"), LLONG_MAX);
+        goto error;
+    }
+
     if (disk->blkdeviotune.total_bytes_sec) {
         virBufferAsprintf(&opt, ",bps=%llu",
                           disk->blkdeviotune.total_bytes_sec);