aarch64: update RegList length to 500
aarch64: update RegList length to 500

This is to match the upstream kvm-bindings max_len() of
500 elements.

This was not previously failing because vmm-sys-util
version prior to 0.8.0 was not checking the number
of elements to be smaller than the max length.

Signed-off-by: alindima <120e67a722c5477956e37ee6f342b2830138960b@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker