KVM: arm64: Document ordering requirements for irqbypass

One of the not-so-obvious requirements for restoring a VM is ensuring
that the vITS has been restored _before_ creating any irqbypass mappings.
This is because KVM needs to get the guest translation for MSIs to
correctly assemble the vLPI mapping getting installed in the physical
ITS.

Document the restore ordering requirements necessary for GICv4 vLPI
injection to work.

Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250226183124.82094-5-oliver.upton@linux.dev


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