Allow --qemu and file:// --mirrors to be used together

When using file:// mirrors, rootfs.py will spawn a local HTTP
server at 127.0.0.1.

In combination with --qemu, this poses a problem: downloads
on the host machine will work, but downloads on the guest
machine will not reach the local server.

This commit introduces a change to rewrite the address to
10.0.2.2 inside the guest, only when QEMU=True is set in
the configuration, allowing this combination to build
past the get_network improvement step.

steps/helpers.sh

@@ -283,6 +283,10 @@ download_source_line() {
     fname="${fname:-$(basename "${upstream_url}")}"
     if ! [ -e "${fname}" ]; then
         for mirror in $(randomize "${MIRRORS}"); do
+            # In qemu SimpleMirror is not running on the guest os, use qemu IP
+            case "${QEMU}-${mirror}" in 'True-http://127.0.0.1'*)
+                mirror="http://10.0.2.2${mirror#'http://127.0.0.1'}"
+            esac
             mirror_url="${mirror}/${fname}"
             echo "${mirror_url}"
             curl --fail --retry 3 --location "${mirror_url}" --output "${fname}" || true && break