custom_vm.md: Add instructions for serial console am: 021c081034 am: e0bd88b032

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Virtualization/+/3135558

Change-Id: I87429518fe5ffee482e0b62c04d88e3c96308390
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>