Add unit type to help text for memory.

It's not obvious that the specified memory in calls to configure-vm is
specified in KB. Non-obvious errors often result if you try to boot a vm
with extremly small amounts of memory. Making the help text more
specific can help to avoid mistakes.

Change-Id: Ia493445465de69e09dc6d14d27c5206a31c0b1a6
Merge "Add unit type to help text for memory."