Create disk images without root

[eduardosm]

mke2fs has a -d option that allows to populate the newly created filesystem without needing to temporarily mount it. That allows to use parted and mkfs.ext3 on regular files without needing root access.

I also fixed some issues I bumped into while testing (see commits).

[Googulator]

A quick note: this should not affect probably the most common case of qemu or bare-metal with kernel bootstrap without --external-sources, which already proceeds without ever creating an FS image other than srcfs.