KVM: arm64: Get sys_reg encoding from descriptor in idregs_debug_show()

KVM: arm64: Get sys_reg encoding from descriptor in idregs_debug_show()

KVM is about to add support for more VM-scoped feature ID regs that
live outside of the id_regs[] array, which means the index of the
debugfs iterator may not actually be an index into the array.

Prepare by getting the sys_reg encoding from the descriptor itself.

Reviewed-by: Sebastian Ott <sebott@redhat.com>
Link: https://lore.kernel.org/r/20240619174036.483943-2-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>