Fix virsh snapshot-list error reporting
Fix virsh snapshot-list error reporting

Noticed that the expected "not supported" error is dropped when
invoking 'virsh snapshot-list dom' on a Xen installation running
the libxl driver

 virsh snapshot-list test
 error: Invalid snapshot: virDomainSnapshotFree

The error is overwritten by a call to virDomainSnapshotFree
in cleanup code within cmdSnapshotList.  Prevent overwritting
the real error by not calling virDomainSnapshotFree with a NULL
virDomainSnapshotPtr.

olafhering/libvirt,cbosdo/libvirt,datto/libvirt,zhlcindy/libvirt-1.1.4-maintain,agx/libvirt,rlaager/libvirt,datto/libvirt,agx/libvirt,agx/libvirt,shugaoye/libvirt,zippy2/libvirt,zippy2/libvirt,cbosdo/libvirt,elmarco/libvirt,cbosdo/libvirt,andreabolognani/libvirt,eskultety/libvirt,VenkatDatta/libvirt,datto/libvirt,jfehlig/libvirt,zippy2/libvirt,rlaager/libvirt,cbosdo/libvirt,zhlcindy/libvirt-1.1.4-maintain,jardasgit/libvirt,andreabolognani/libvirt,jfehlig/libvirt,shugaoye/libvirt,shugaoye/libvirt,agx/libvirt,andreabolognani/libvirt,nertpinx/libvirt,shugaoye/libvirt,crobinso/libvirt,zhlcindy/libvirt-1.1.4-maintain,andreabolognani/libvirt,taget/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,elmarco/libvirt,olafhering/libvirt,libvirt/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,eskultety/libvirt,jfehlig/libvirt,elmarco/libvirt,shugaoye/libvirt,taget/libvirt,fabianfreyer/libvirt,taget/libvirt,taget/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,libvirt/libvirt,libvirt/libvirt,zhlcindy/libvirt-1.1.4-maintain,nertpinx/libvirt,elmarco/libvirt,olafhering/libvirt,jardasgit/libvirt,datto/libvirt,crobinso/libvirt,eskultety/libvirt,fabianfreyer/libvirt,eskultety/libvirt,zhlcindy/libvirt-1.1.4-maintain,rlaager/libvirt,VenkatDatta/libvirt,datto/libvirt,taget/libvirt,andreabolognani/libvirt,olafhering/libvirt,jardasgit/libvirt,nertpinx/libvirt,rlaager/libvirt,VenkatDatta/libvirt,cbosdo/libvirt,agx/libvirt,libvirt/libvirt,nertpinx/libvirt,crobinso/libvirt,fabianfreyer/libvirt,crobinso/libvirt,zippy2/libvirt,VenkatDatta/libvirt,rlaager/libvirt,elmarco/libvirt,eskultety/libvirt,nertpinx/libvirt