tests: Prefer xorriso over genisoimage to generate test.iso

This Debian page explains the upstream situation:
https://wiki.debian.org/genisoimage

On Fedora, xorriso provides a compatibility program called "mkisofs".
However this is not present in Debian.  Hence the choice to look for
the program called "xorrisofs".

docs/guestfs-building.pod

@@ -152,9 +152,9 @@ I<Required>.
 
 I<Required>.
 
-=item genisoimage
+=item xorriso, genisoimage or mkisofs
 
-I<Required>.
+One of these is I<Required>.
 
 =item libxml2
 

m4/guestfs-progs.m4

@@ -49,10 +49,10 @@ AC_CHECK_PROG([GPERF],[gperf],[gperf],[no])
 test "x$GPERF" = "xno" &&
     AC_MSG_ERROR([gperf must be installed])
 
-dnl Check for genisoimage/mkisofs
-AC_PATH_PROGS([GENISOIMAGE],[genisoimage mkisofs],[no],
+dnl Check for xorriso/genisoimage/mkisofs.
+AC_PATH_PROGS([MKISOFS],[xorrisofs genisoimage mkisofs],[no],
     [$PATH$PATH_SEPARATOR/usr/sbin$PATH_SEPARATOR/sbin])
-test "x$GENISOIMAGE" = "xno" && AC_MSG_ERROR([genisoimage must be installed])
+test "x$MKISOFS" = "xno" && AC_MSG_ERROR([xorriso or genisoimage or mkisofs must be installed])
 
 dnl Check for optional xmllint.
 AC_CHECK_PROG([XMLLINT],[xmllint],[xmllint],[no])

test-data/Makefile.am

@@ -98,6 +98,6 @@ test.iso: $(images_files)
 	cp $(image_files) d/
 	mkdir -p d/directory
 	cd d && ln -sf /10klines abssymlink
-	cd d && $(GENISOIMAGE) -J -r -o ../$@-t .
+	cd d && $(MKISOFS) -J -r -o ../$@-t .
 	rm -rf d
 	mv $@-t $@