vm: change how commands can be easily run with a normal user account on the guest

9p has no option to map uid/gid, which means that the user on the guest should
have the same ids than on the host to avoid issues when accessing mount points.

This is easier to create such user in 'vm.run' rather than in 'vm.install-*'.
In other words, this commit reverts to the previous behaviour that
'vm.install-*' does not create a normal user account by default.