Only assign newDef when we have a new def.

Only assign newDef when we have a new def.

While playing around with def/newDef with the qemu code,
I noticed that newDef was *always* getting set to a value,
even when I didn't redefine the domain.  I think the problem
is the virDomainLoadConfig is always doing virDomainAssignDef
regardless of whether the domain already exists in the hashtable.
In turn, virDomainAssignDef is assigning the definition (which
is actually a duplicate) to newDef.  Fix this so that newDef stays
NULL until we actually have a new def.

Signed-off-by: Chris Lalancette <clalance@redhat.com>