Fix the guest resolution agent so it doesn't crash on first boot.

Right now the agent just crashes because xenstore is not there yet
on the reboot after intsalling the tools. Now it gives a friendly
message and shuts down gracefully.

OXT-44

Signed-off-by: Ross Philipson <ross.philipson@gmail.com>
Merge pull request #3 from rossphilipson/resolution_crash

Fix the guest resolution agent so it doesn't crash on first boot.