initrdinject: Force added files to be owned as root (bz 1686464)

systemd in fedora30 has some new security restriction about non-root
owned directories. Initrd inject would tickle this because the cpio
archive would cause the root dir in the initrd to be owned by the
uid that launched virt-install.

Pass --owner=+0:+0 to cpio to force root ownership

Suggested-by: James Szinger <jszinger@gmail.com>

https://bugzilla.redhat.com/show_bug.cgi?id=1686464

virtinst/initrdinject.py

@@ -31,7 +31,7 @@ def perform_initrd_injections(initrd, injections, scratchdir):
                                  stderr=subprocess.PIPE,
                                  cwd=tempdir)
     cpio_proc = subprocess.Popen(['cpio', '--create', '--null', '--quiet',
-                                  '--format=newc'],
+                                  '--format=newc', '--owner=+0:+0'],
                                  stdin=find_proc.stdout,
                                  stdout=subprocess.PIPE,
                                  stderr=subprocess.PIPE,