vpick: use prefix_roota() to avoid double slash in log messages

vpick: use prefix_roota() to avoid double slash in log messages

If the toplevel_path is empty we end up with doubled leading slash,
which looks weird:

[ 4737.028985] testsuite-74.sh[102]: Inode '//var/lib/machines/mytree.v/mytree_37.0_arm64+2-3' has wrong type, found 'dir'.
[ 4737.028985] testsuite-74.sh[102]: Failed to pick version for '/var/lib/machines/mytree.v': Is a directory
...
[ 4316.957536] testsuite-74.sh[99]: Failed to open '//var/lib/machines/mytree.v/mytree_37.0': No such file or directory
...