B #75: Check that network-scripts are actually present in guest

Use same method to detect presence if system is capable of NETCFG_TYPE=scripts as in netcfg-scripts - by checking for presence of -x /etc/sysconfig/network-scripts/ifup, not just directory -d /etc/sysconfig/network-scripts/ which exists, because OS has put placeholder text file, describing that given configuration method is deprecated by OS.

Co-authored-by: Marcis Lielturks <marcis.lielturks@bite.lv>