Pass inst.disklabel=mbr for ppc64le virt-install image builds

Otherwise they don't boot, see:
https://bugzilla.redhat.com/show_bug.cgi?id=2209760
This is fixed from F39 onwards (it will use MBR disklabels for
ppc64le installs by default), but since we're still building F37
and F38 disk images now, we need it here.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

createhdds.py

@@ -321,6 +321,11 @@ class VirtInstallImage(object):
                 loctmp = "https://download.fedoraproject.org/pub/{0}/releases/{1}/{2}/{3}/os/"
             ksfile = self.kickstart_file
             xargs = "inst.ks=file:/{0}".format(ksfile)
+            if arch == "ppc64le":
+                # https://bugzilla.redhat.com/show_bug.cgi?id=2209760
+                # FIXME: can be dropped when we no longer create F38
+                # images for ppc64le
+                xargs += " inst.disklabel=mbr"
             args = ["virt-install", "--disk", "size={0},path={1}".format(self.size, tmpfile),
                     "--os-variant", shortid, "-x", xargs, "--initrd-inject",
                     "{0}/{1}".format(SCRIPTDIR, ksfile), "--location",