promoting virtualbox-dkms to virtualbox pre-depends

[Gianfranco Costamagna <costamagnagianfranco@yahoo.it>]

As shown in policy 7.2

"You should not specify a Pre-Depends entry for a package before this has been discussed on the debian-devel mailing
list and a consensus about doing that has been reached. See Dependencies, Section 3.5."

the problem actually is that virtualbox-dkms should be configured *before* configuring virtualbox, otherwise
without a built kernel module, restarting VMs
will fail and lead to an half-configured package.

Please see bugs #798527 and #798979 as examples.

(this is a subpackage depending on another subpackage that belongs to the same src, I don't get why I should discuss such internal
changes, but well, policy is policy)



cheers,

Gianfranco