Interrupt remapping to PIRQs in HVM guests

Interrupt remapping to PIRQs in HVM guests

This patch allows HVM guests to remap interrupts and MSIs into pirqs;
once the mapping is in place the guest will receive the interrupt (or
the MSI) as an event.  The interrupt to be remapped can either be an
interrupt of an emulated device or an interrupt of a passthrough
device and we keep track of that.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>