Add functions implementing MSI-X support. First user will be virtio-pci.
Note that platform must set a flag to declare MSI supported: this
is a safety measure to avoid breaking platforms which should support
MSI-X but currently lack this in the interrupt controller emulation.
For PC this will be set by APIC.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>