Check if qemu-img is available
Add CheckAptProxy task

Check if the specified APT proxy server can be reached, informing the
user if this can't be done. That will help them to debug the errors that
will be raised by `apt-get` because of the misleading proxy
configuration.

This closes #95.
Might as well check apt proxy early
Moved CheckAptProxy to preparation phase
Changed the logging to output a warning instead of an error
Changed the error message a little