Adds functions to read and write state of devices in vmd.

This is required for implementing vmctl send and vmctl receive. vmctl
send / receive are two new options that will support snapshotting VMs
and migrating VMs from one host to another. The atomicio files are
copied from usr.bin/ssh.

Patch from Pratik Vyas; this project was undertaken at San Jose State
University along with his three teammates, Ashwin, Harshada and Siri
with mlarkin@ as the advisor.

OK mlarkin@