xenconfig: add support for multiple USB devices syntax

In Xen>=4.3, libxl supports new syntax for USB devices:
usbdevice=[ "DEVICE", "DEVICE", ... ]
Add support for that in xenconfig driver. When only one device is
defined, keep using old syntax for backward compatibility.

Adjust tests for changed options order.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>

tests/xmconfigdata/test-fullvirt-usbtablet.cfg

@@ -14,8 +14,6 @@ on_poweroff = "destroy"
 on_reboot = "restart"
 on_crash = "restart"
 device_model = "/usr/lib/xen/bin/qemu-dm"
-usb = 1
-usbdevice = "tablet"
 sdl = 0
 vnc = 1
 vncunused = 1
@@ -25,3 +23,5 @@ vif = [ "mac=00:16:3e:66:92:9c,bridge=xenbr1,script=vif-bridge,model=e1000,type=
 parallel = "none"
 serial = "none"
 disk = [ "phy:/dev/HostVG/XenGuest2,hda,w", "file:/root/boot.iso,hdc:cdrom,r" ]
+usb = 1
+usbdevice = "tablet"