KVM: Convert comment into an assertion in kvm_io_bus_register_dev()

Instead of having a comment indicating the need to hold slots_lock
when calling kvm_io_bus_register_dev(), make it explicit with
a lockdep assertion.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231207151201.3028710-6-maz@kernel.org


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