backend: Avoid IsoPrefix command when no iso attached

Added check to ImagesHandler.cdPathWindowsToLinux to immediately return
when passed empty string and avoid running the IsoPrefixVSDCommand to
the same end result.

Change-Id: If83986270400fdefd31aa01a1736de350ab6a5ec
Signed-off-by: Martin Betak <mbetak@redhat.com>