Stop depending on invirt-mail-config in invirt-base

This was basically an attempt to stuff an XVM site-specific config
into the namespace of invirt packages. If we don't expect other users
of invirt to roll their own packages, we should depend on packages
they can't provide.

The dependency on xvm-mail-config will move into a set of xvm
metapackages

svn path=/trunk/packages/invirt-base/; revision=1803