openvz: Fix regression in config file parsing

openvz: Fix regression in config file parsing

As reported by Diego Blanco in

  https://bugzilla.redhat.com/show_bug.cgi?id=702602

commit f0443765 which replaced openvz_readline to getline(3)
broke OpenVZ driver as it changed semantics of EOF-handling
when parsing OpenVZ configuration.

There're several other issues reported with current OpenVZ driver:

 #1: unclear error message when parsing "CPUS=" line
 #2: openvz driver goes into crashing loop
 #3: "NETIF=" line in configuration is not parsed correctly
 #4: aborts even when optional parameter is missing
 #5: there's a potential memory leak

This updated patch to fix #[145]. This patch does not fix #[23]
as I haven't verified these yet, but this at least got me to run
OpenVZ on libvirt once again.