v2v: Fix getExternalVMs if getting disk size failed

In getExternalVMs we try to report the disk size, but if this info is
not available for certain disk, we continue to collect info about other
disks. Due to incorrect error handling, the request was aborted after
the first error:

      File "/usr/share/vdsm/v2v.py", line 684, in _add_disk_info
        disk['capacity'] = str(capacity)
    UnboundLocalError: local variable 'capacity' referenced before assignment

This patch fixes the error handling so failure to get disk size would
not fail the entire request.

Change-Id: I7e91190a255a4d9bd2f474e125c49b18e3d3bfd4
Reported-by: Ian Fraser <ian.fraser@asm.org.uk>
Bug-Url: https://bugzilla.redhat.com/1265556
Backport-to: 3.6
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: https://gerrit.ovirt.org/46540
Reviewed-by: Francesco Romani <fromani@redhat.com>
Continuous-Integration: Jenkins CI

vdsm/v2v.py

@@ -689,9 +689,9 @@ def _add_disk_info(conn, disk):
             _, capacity, alloc = vol.info()
         except libvirt.libvirtError:
             logging.exception("Error getting disk size")
-
-        disk['capacity'] = str(capacity)
-        disk['allocation'] = str(alloc)
+        else:
+            disk['capacity'] = str(capacity)
+            disk['allocation'] = str(alloc)
 
 
 def _add_disks(root, params):