windows: Increase the virtio timeout to 15min

The original value was 5min but was rather small. In Desktop versions
of windows the VirtIO installation may take longer.
windows: Increase the connection check interval

Start by waiting for 5sec before retrying and double the interval
after each retry.
Fix a 'FileNotFoundError' is not defined error

This exception does not exist in python 2.x
Always use get_command() to import commands

If you are a regular user, some root commands like modprobe may not be
in your path.
Reference disk.file in the begging of snapshot()

If you print the snapshotting message before disk.file is referenced,
you may end up outputting messages like this:

Snapshotting media source ... Examining source media `<image_file>' ...

Instead of:

Examining source media `<image_file>' ... looks like a block device
Snapshotting media source ... done
Add a display attribute in sysprep

This can be used to change the displayed name of the sysprep task.