perl: Don't fail if 'type' (disk format) attribute is missing in libvirt XML (RHBZ#701814).

Old versions of libvirt allowed you to define disks like this:

    <disk type='file' device='disk'>
      <driver name='qemu'/>
      ...

Since the <driver> element does not have a 'type' attribute (which
defines the format), we are supposed to do autodetection, so the
format should be undefined.

However what actually happened was that the code in
Sys::Guestfs::Lib::open_guest received format as an empty string from
the xpath query, causing libguestfs to give an error.

If the xpath query returns the format as an empty string, undefine it.

1 files changed, 5 insertions, 1 deletions

diff --git a/perl/lib/Sys/Guestfs/Lib.pm b/perl/lib/Sys/Guestfs/Lib.pm
index 42d2e819..2ccc09af 100644
--- a/perl/lib/Sys/Guestfs/Lib.pm
+++ b/perl/lib/Sys/Guestfs/Lib.pm
@@ -234,7 +234,11 @@ sub open_guest
 
             # Get the disk format (may not be set).
             my $format = $p->find ('./driver/@type', $node);
-            $format = $format->to_literal if $format;
+            if ($format) {
+                $format = $format->to_literal;
+            } else {
+                undef $format; # RHBZ#701814.
+            }
 
             push @disks, [ $filename, $format ];
         }