virtme-run: Refactor virtfs export code

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Initial support for --root

This is rather ugly.  For now, it's automatically mapping the virtme
install directory.  I think it should instead package virtme-init
in the initramfs.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Add --busybox to override the path to busybox

This makes is possible to do hackish cross-arch runs using
--busybox and --root.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>