tests/avocado: Test Xen guest support under KVM

tests/avocado: Test Xen guest support under KVM

Exercise guests with a few different modes for interrupt delivery. In
particular we want to cover:

 • Xen event channel delivery via GSI to the I/O APIC
 • Xen event channel delivery via GSI to the i8259 PIC
 • MSIs routed to PIRQ event channels
 • GSIs routed to PIRQ event channels

As well as some variants of normal non-Xen stuff like MSI to vAPIC and
PCI INTx going to the I/O APIC and PIC, which ought to still work even
in Xen mode.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>