vmimage.subr: Log pkg/local.sqlite if pkgbase

We were doing this in vm_extra_install_packages but VM images without
any extra packages installed would not get this installed. This
results in a pkgbase system which thinks it doesn't have any packages
installed (even though all the files are right there).

Add a "metalog_add_data ./var/db/pkg/local.sqlite" call to the pkgbase
install code path, and make the call from vm_extra_install_packages
conditional on !PKGBASE.

Reported by: Michael Dexter
MFC after: immediately (needed for 15.0-RC1)