KVM: x86: Drop semi-arbitrary restrictions on IPI type in fastpath

KVM: x86: Drop semi-arbitrary restrictions on IPI type in fastpath

Drop the restrictions on fastpath IPIs only working for fixed IRQs with a
physical destination now that the fastpath is explicitly limited to "fast"
delivery.  Limiting delivery to a single physical APIC ID guarantees only
one vCPU will receive the event, but that isn't necessary "fast", e.g. if
the targeted vCPU is the last of 4096 vCPUs.  And logical destination mode
or shorthand (to self) can also be fast, e.g. if only a few vCPUs are
being targeted.  Lastly, there's nothing inherently slow about delivering
an NMI, INIT, SIPI, SMI, etc., i.e. there's no reason to artificially
limit fastpath delivery to fixed vector IRQs.

Link: https://lore.kernel.org/r/20250805190526.1453366-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>