This is the official way, needs no own parsing (therefore less
fault-prone) and shorter. Also fixes Debian bug #560011 -- the manual
parsing did not catch all syntax variants.
Also add some comments before /etc/apt/sources.list generation in the
Ubuntu hooks.
Temporarily use resolv.conf from dom0 when chrooting into domU,
to be able to run apt-get and yum inside chroot.
This permits to use different nameservers in domU, and does not
break install nor offline update from dom0.
