[virtualization] Print default memory in help

In the command line usage string, print the default memory for the the
VMM.

Change-Id: Ic5f32be8f16a12c75481de27b86c5054bf933164
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/450319
Testability-Review: Abdulla Kamar <abdulla@google.com>
Fuchsia-Auto-Submit: Abdulla Kamar <abdulla@google.com>
Reviewed-by: Alex Legg <alexlegg@google.com>
Commit-Queue: Abdulla Kamar <abdulla@google.com>