df: Avoid a compilation error if libxml2 is not available.

virt_df-domains.o: In function `add_domain':
/home/feeliwood/Downloads/libguestfs-1.17.21/df/domains.c:274: undefined reference to `guestfs___for_each_disk'

guestfs___for_each_disk is only available when libvirt _and_ libxml2
dependencies are available at compile time.

Thanks Tho Huynh.

df/domains.c

@@ -36,7 +36,7 @@
 #include "options.h"
 #include "virt-df.h"
 
-#ifdef HAVE_LIBVIRT
+#if defined(HAVE_LIBVIRT) && defined(HAVE_LIBXML2)
 
 /* Limit the number of devices we will ever add to the appliance.  The
  * overall limit in current libguestfs is 25: 26 = number of letters

df/main.c

@@ -260,10 +260,10 @@ main (int argc, char *argv[])
    * the full list of guests and drives, which we add in batches.
    */
   if (drvs == NULL) {
-#ifdef HAVE_LIBVIRT
+#if defined(HAVE_LIBVIRT) && defined(HAVE_LIBXML2)
     get_domains_from_libvirt ();
 #else
-    fprintf (stderr, _("%s: compiled without support for libvirt.\n"),
+    fprintf (stderr, _("%s: compiled without support for libvirt and/or libxml2.\n"),
              program_name);
     exit (EXIT_FAILURE);
 #endif

df/virt-df.h

@@ -31,7 +31,7 @@ extern int uuid;                /* --uuid */
 extern int df_on_handle (const char *name, const char *uuid, char **devices, int offset);
 
 /* domains.c */
-#ifdef HAVE_LIBVIRT
+#if defined(HAVE_LIBVIRT) && defined(HAVE_LIBXML2)
 extern void get_domains_from_libvirt (void);
 #endif