KVM: x86: Decouple device assignment from IRQ bypass

KVM: x86: Decouple device assignment from IRQ bypass

Use a dedicated counter to track the number of IRQs that can utilize IRQ
bypass instead of piggybacking the assigned device count.  As evidenced by
commit 2edd9cb79fb3 ("kvm: detect assigned device via irqbypass manager"),
it's possible for a device to be able to post IRQs to a vCPU without said
device being assigned to a VM.

Leave the calls to kvm_arch_{start,end}_assignment() alone for the moment
to avoid regressing the MMIO stale data mitigation.  KVM is abusing the
assigned device count when applying mmio_stale_data_clear, and it's not at
all clear if vDPA devices rely on this behavior.  This will hopefully be
cleaned up in the future, as the number of assigned devices is a terrible
heuristic for detecting if a VM has access to host MMIO.

Link: https://lore.kernel.org/r/20250611224604.313496-55-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>