KVM: Fix buffer overflow in kvm_set_irq()

KVM: Fix buffer overflow in kvm_set_irq()

Bugdb: 13966
kvm_set_irq() has an internal buffer of three irq routing entries, allowing
connecting a GSI to three IRQ chips or on MSI.  However setup_routing_entry()
does not properly enforce this, allowing three irqchip routes followed by
an MSI route to overflow the buffer.

Fix by ensuring that an MSI entry is added to an empty list.
This fixes: CVE-2012-2137
Signed-off-by: Avi Kivity <avi@redhat.com>