* src/qemu/qemu_migration.c: if 'vmdef' is NULL, the function
  virDomainSaveConfig still dereferences it, it doesn't make
  sense, so should add return value check to make sure 'vmdef'
  is non-NULL before calling virDomainSaveConfig, in addition,
  in order to debug later, also should record error information
  into log.
Signed-off-by: NAlex Jia <ajia@redhat.com>