KVM: arm64: Consolidate idreg callbacks

Most of the ID_DESC() users use the same callbacks, with only a few
overrides. Consolidate the common callbacks in a macro, and consistently
use it everywhere.

Whilst we're at it, give ID_UNALLOCATED() a .name string, so that we can
easily decode traces.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/r/20250220134907.554085-7-maz@kernel.org


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