Warning: program compiled against libxml 209 using older 207
It seems to be a warning and not an error so you don't have to take it so seriously :)
Besides, in your sources.list
you should not be putting those extra repo since with 12.04 (precise) you should be ok. Those extra repos can cause a lot of trouble and interdependency problems if you don't take care.
So, I recommend you to only left your sources.list
with the only needed for your distribution:
deb http://download.virtualbox.org/virtualbox/debian precise contrib
Also, you can try to do a package upgrade with sudo apt-get update
or sudo apt-get dist-upgrade
to see if the version issue dissapear with a libxml
upgrade.
Also, you can try to force an update on libxml
by issuing sudo apt-get install libxml
to see if that solves your warning.
HTH