passthrough/io: fall back to remapping interrupt when we can't use VT-d PI

passthrough/io: fall back to remapping interrupt when we can't use VT-d PI

The current logic of using VT-d pi is when guest configurates the pirq's
destination vcpu to a single vcpu, the according IRTE is updated to
posted format. If the destination of the pirq is multiple vcpus, we will
stay in posted format. Obviously, we should fall back to remapping interrupt
when guest wrongly configurate destination of pirq or makes it have
multi-destination vcpus.

Signed-off-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
[jb: guard against vcpu being NULL]
Reviewed-by: Jan Beulich <jbeulich@suse.com>