KVM: x86: Assert that non-MSI doesn't have bypass vCPU when deleting producer

When disconnecting a non-MSI irqfd from an IRQ bypass producer, WARN if
the irqfd is configured for IRQ bypass and set its IRTE back to remapped
mode to harden against kernel/KVM bugs (keeping the irqfd in bypass mode
is often fatal to the host).

Deactivating an irqfd (removing it from the list of irqfds), updating
irqfd routes, and the code in question are all mutually exclusive (all
run under irqfds.lock).  If an irqfd is configured for bypass, and the
irqfd is deassigned at the same time IRQ routing is updated (to change the
routing to non-MSI), then either kvm_arch_update_irqfd_routing() should
process the irqfd routing change and put the IRTE into remapped mode
(routing update "wins"), or kvm_arch_irq_bypass_del_producer() should see
the MSI routing info (deactivation "wins").

Link: https://patch.msgid.link/20260113174606.104978-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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