kvm tools: Fix running with --initrd and no real discs/rootfs

It's useful to boot with no -d and no implicit 9p root, with --initrd.
Since commit eaf720b285947a6f4e29174d0eba1899de31d8ab, we die() failing
to create the 2nd stage custom rootfs.  This patch doesn't try to set up
a custom rootfs by default if user specifies an initrd on the commandline.

Signed-off-by: Matt Evans <matt@ozlabs.org>
Signed-off-by: Pekka Enberg <penberg@kernel.org>