xenserver: Keep Centos network config files up-to-date even with vswitch.

xenserver: Keep Centos network config files up-to-date even with vswitch.

When the vswitch is running, it doesn't care about, doesn't need, and
doesn't use the files in /etc/sysconfig/network-scripts.  But if you ever
remove the vswitch (e.g. "rpm -e vswitch") then the regular bridge code
does need them.  There is no guarantee that we will be able to get access
to the xapi database at "rpm -e" time, so it is better to keep these
configuration files up-to-date after each operation.  This commit does
that.

Given that we keep these files up-to-date after each operation now, there
is no need to call interface-reconfigure at RPM uninstall time, so this
commit also removes the script code that did that.