KVM: arm64: vgic-v4: Fall back to software irqbypass if LPI not found

Continuing with the theme of broken VMMs and guests, irqbypass
registration can fail if the virtual ITS lacks a translation for the
MSI. Either the guest hasn't mapped it or userspace may have forgotten
to restore the ITS.

Exit silently and allow irqbypass configuration to succeed. As a reward
for ingenuity, LPIs are demoted to software injection.

Tested-by: Sudheer Dantuluri <dantuluris@google.com>
Fixes: 196b1364 ("KVM: arm/arm64: GICv4: Wire mapping/unmapping of VLPIs in VFIO irq bypass")
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250226183124.82094-4-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>