ramdisk_node_list: Add urandom node

Bionic requires random numbers to init the shadow call stack. Those
numbers are obtained via the syscall getrandom (non-blocking) and will
fallback to /dev/urandom if the former fails.

When loading pKVM modules, we are so early in the boot process that the
only source of entropy for the linux RNG are the architecture random
number generators... which might be available on some platforms. Without
any source of entropy, the only way of generating a random number is to
try to generate some, which is what the bionic fallback expects via
urandom.

As a consequence, add the urandom node to the initramfs.

Bug: 274876849
Change-Id: I164b08f026a238dad9f27a345bdef96717f2aa74