Filter proxy_irrs from proxied interrupts from the host to only signal bits the guest has configured and is expecting

[cperezvargas]

When the hypervisor presents a proxy interrupt intercept message, the kernel directly puts the signaled bits into proxy_irr without any validation. Today, the vp backing implementations for both SNP and TDX trust that the hypervisor is only signaling bits that the guest has requested.

We can't rely on that, as a malicious hypervisor may choose to signal any arbitrary bits. Instead, underhill needs to track which SINTs have been enabled by the guest via hypercalls and SINT MSR writes, and only allow forwarding those to the APIC.