qemu.tests.usb: Disable xhci on old RHEL guests

These RHEL guests older than RHEL5 aren't shipped
with xhci driver. Disable xhci testing on them.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
qemu.tests.usb: Disable usb_audio on RHEL7 host

The RHEL7 host doesn't have usb_audio support now.
Disable it.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>