UBUNTU: [Packaging]: download virtualbox from sources

UBUNTU: [Packaging]: download virtualbox from sources

BugLink: https://bugs.launchpad.net/bugs/1848788
dkms packages are gone from the archive when new versions are uploaded.
dkms-build grab them from the existing sources on the builder, which will
work when the expected versions are present on any of the configured
sources.

virtualbox, however, right now, only grabs it from the main archive,
preventing builds when they have been superseded. Fix it by giving the
relative pool address instead of the absolute URL for the archive.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Andrea Righi <andrea.righi@canonical.com>
Acked-by: Andy Whitcroft <andy.whitcroft@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>