Fixes HyperV VM Console Log

This patch intends to fix some small nits introduced along with
the Nova console log patch.

The size of the console log can get bigger than expected because
of a small nit when checking the existing log file size as well
as a wrong size constant.

The method which gets the serial port pipe at the moment returns
a list which contains at most one element being the actual pipe
path. In order to avoid confusion this should return the pipe path
or None instead of a list.

Change-Id: I5ce9d1bea572f9098971990d011fa97ebf09233e
Closes-Bug: #1363899
Merge "Fixes HyperV VM Console Log"