qemu: do not crash on USB address with no port and invalid bus

Properly error out when the user requests a port from a bus
that does not have a controller present in the domain XML.

https://bugzilla.redhat.com/show_bug.cgi?id=1441589
(cherry picked from commit b003b978)

src/conf/domain_addr.c

@@ -2009,7 +2009,8 @@ virDomainUSBAddressAssign(virDomainUSBAddressSetPtr addrs,
 
     if (info->type == VIR_DOMAIN_DEVICE_ADDRESS_TYPE_USB) {
         VIR_DEBUG("A USB port on bus %u was requested", info->addr.usb.bus);
-        if (!addrs->buses[info->addr.usb.bus]) {
+        if (info->addr.usb.bus >= addrs->nbuses ||
+            !addrs->buses[info->addr.usb.bus]) {
             virReportError(VIR_ERR_XML_ERROR,
                            _("USB bus %u requested but no controller "
                              "with that index is present"), info->addr.usb.bus);

tests/qemuxml2argvdata/qemuxml2argv-usb-bus-missing.xml

+<domain type='qemu'>
+  <name>QEMUGuest1</name>
+  <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
+  <memory unit='KiB'>219136</memory>
+  <currentMemory unit='KiB'>219136</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='i686' machine='pc'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <devices>
+    <emulator>/usr/bin/qemu-system-i686</emulator>
+    <controller type='usb' index='0'/>
+    <memballoon model='virtio'/>
+    <input type='mouse' bus='usb'>
+      <address type='usb' bus='0'/>
+    </input>
+    <hub type='usb'>
+      <address type='usb' bus='1'/>
+    </hub>
+  </devices>
+</domain>

tests/qemuxml2argvtest.c

@@ -1343,6 +1343,9 @@ mymain(void)
     DO_TEST("usb-port-missing",
             QEMU_CAPS_CHARDEV, QEMU_CAPS_USB_HUB,
             QEMU_CAPS_NODEFCONFIG);
+    DO_TEST_PARSE_ERROR("usb-bus-missing",
+                        QEMU_CAPS_CHARDEV, QEMU_CAPS_USB_HUB,
+                        QEMU_CAPS_NODEFCONFIG);
     DO_TEST("usb-ports",
             QEMU_CAPS_CHARDEV, QEMU_CAPS_USB_HUB,
             QEMU_CAPS_NODEFCONFIG);