Fix for problems reported by Alejandro Hernández:
. VM unmap: handle case where there is no nextvr
Fixes for problems found by running Melkor ELF fuzzing tool:
. VM: better handle case where region prealloc fails by
freeing memory that was allocated so far
. MFS fs_readwrite: EOF check should happen for read and
peek requests, not just read
This fixes#4.
Change-Id: I2adf4eebdfb4c48a297beff0478eed5c917a53a4