pass-through: update IRTE according to guest interrupt config changes

pass-through: update IRTE according to guest interrupt config changes

When guest changes its interrupt configuration (such as, vector, etc.)
for direct-assigned devices, we need to update the associated IRTE
with the new guest vector, so external interrupts from the assigned
devices can be injected to guests without VM-Exit.

For lowest-priority interrupts, we use vector-hashing mechamisn to find
the destination vCPU. This follows the hardware behavior, since modern
Intel CPUs use vector hashing to handle the lowest-priority interrupt.

For multicast/broadcast vCPU, we cannot handle it via interrupt posting,
still use interrupt remapping.

Signed-off-by: Feng Wu <feng.wu@intel.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>