[refresh] Don't resolve URL in zypp_is_changeable_media

This avoids a network roundtrip and fixes a crash related to an uncaught
zypp::PluginScriptException (the alternative would be to handle the
PluginScriptException in zypp_is_changeable_media(), but as CD/DVDs
are not our top priority for distributing nemo, just handle it quickly.
Merge pull request #20 from thp/fix-media-manager-crash

[refresh] Don't resolve URL in zypp_is_changeable_media