[libvirt] new qemu tests require xen support

[Jim Meyering]

Hi Dan,

On a system with stock rawhide kernel (not the xen one), I get two
new test failures because virCapabilitiesSupportsGuestOSType(caps, "xen")
returns 0.

$ ./qemuxml2xmltest
 1) QEMU XML-2-XML minimal                                            ... OK
 2) QEMU XML-2-XML boot-cdrom                                         ... OK
 3) QEMU XML-2-XML boot-network                                       ... OK
 4) QEMU XML-2-XML boot-floppy                                        ... OK
 5) QEMU XML-2-XML bootloader                                         ... libvir: QEMU error : unknown OS type xen
FAILED
 6) QEMU XML-2-XML clock-utc                                          ... OK
 7) QEMU XML-2-XML clock-localtime                                    ... OK
 8) QEMU XML-2-XML disk-cdrom                                         ... OK
 9) QEMU XML-2-XML disk-floppy                                        ... OK
10) QEMU XML-2-XML disk-many                                          ... OK
11) QEMU XML-2-XML disk-xenvbd                                        ... OK
12) QEMU XML-2-XML graphics-vnc                                       ... OK
13) QEMU XML-2-XML graphics-sdl                                       ... OK
14) QEMU XML-2-XML input-usbmouse                                     ... OK
15) QEMU XML-2-XML input-usbtablet                                    ... OK
16) QEMU XML-2-XML input-xen                                          ... libvir: QEMU error : unknown OS type xen
FAILED
17) QEMU XML-2-XML misc-acpi                                          ... OK
18) QEMU XML-2-XML misc-no-reboot                                     ... OK
19) QEMU XML-2-XML net-user                                           ... OK
20) QEMU XML-2-XML net-virtio                                         ... OK
21) QEMU XML-2-XML serial-vc                                          ... OK
22) QEMU XML-2-XML serial-pty                                         ... OK
23) QEMU XML-2-XML serial-dev                                         ... OK
24) QEMU XML-2-XML serial-file                                        ... OK
25) QEMU XML-2-XML serial-unix                                        ... OK
26) QEMU XML-2-XML serial-tcp                                         ... OK
27) QEMU XML-2-XML serial-udp                                         ... OK
28) QEMU XML-2-XML serial-tcp-telnet                                  ... OK
29) QEMU XML-2-XML serial-many                                        ... OK
30) QEMU XML-2-XML parallel-tcp                                       ... OK
31) QEMU XML-2-XML console-compat                                     ... OK
[Exit 1]

qemuxml2argvtest fails the same way.