installerinject: Check for genisoimage and mkisofs before calling it

Let's check for both genisoimage and mkisofs as some distros will ship
mkisofs without linking it to genisoimage, as OpenSUSE Leap 15.1 does.

And, I know, genisoimage is a requirement of the project. However, it's
not packaged on OpenSUSE Leap 15.1 while mkisofs is present there.

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>

virtinst/install/installerinject.py

@@ -45,7 +45,15 @@ def _run_initrd_commands(initrd, tempdir):
 
 
 def _run_iso_commands(iso, tempdir, cloudinit=False):
-    cmd = ["genisoimage",
+    # Some distros do not link mkisofs to genisoimage (or vice-versa). As a
+    # result of this, we have to actually check for both programs and use the
+    # most appropriate one.
+    programs = ["genisoimage", "mkisofs"]
+    for program in programs:
+        if shutil.which(program):
+            break
+
+    cmd = [program,
            "-o", iso,
            "-J",
            "-input-charset", "utf8",