Fix AbsoluteCounterPosition to match UPnP spec
Fix AbsoluteCounterPosition to match UPnP spec

UPnP AV Transport Specification v1.01 states that the
AbsoluteCounterPosition state variable is a signed value, where as the
xml document has it as an unsigned value.  Rygel stores the
AbsoluteCounterPosition internally as an signed value.

https://bugzilla.gnome.org/show_bug.cgi?id=709190

GNOME/rygel,Tarnyko/rygel,Pelagicore/Media-Manager-Rygel-patched,GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched,Tarnyko/rygel,GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched