nspawn: don't try mknod() of /dev/console with the correct major/minor

nspawn: don't try mknod() of /dev/console with the correct major/minor

We overmount /dev/console with an external pty anyway, hence there's no
point in using the real major/minor when we create the node to
overmount. Instead, use the one of /dev/null now.

This fixes a race against the cgroup device controller setup we are
using. In case /dev/console was create before the cgroup policy was
applied all was good, but if created in the opposite order the mknod()
would fail, since creating /dev/console is not allowed by it. Creating
/dev/null instances is however permitted, and hence use it.