Comment 36 for bug 1913421

From debugging it seems Bionic didn't have CONFIG_MODULE_UPGRADES set correctly.
(gdb) p dirs
$1 = {0x0, 0x0, 0x0}
Is the latter of:
  168 #ifdef CONFIG_MODULE_UPGRADES
  169 char *version_dir;
  170 char *dirs[4];
  171 #else
  172 char *dirs[3];
  173 #endif

It turned out that on rebase the change "d/rules: --enable-module-upgrades not needed for qemu-system-x86-xen" silently auto-applied to disable the option on the main qemu build (the -xen pkg didn't exist back then).
=> Fixed

Building for the next test iteration now ...