qemu: catch failure of drive_add
    
    Previously when QEMU failed "drive_add" due to an error opening
    a file it would report
    
      "could not open disk image"
    
    These days though, QEMU reports
    
      "Could not open '/tmp/virtd-test_e3hnhh5/disk1.qcow2': Permission denied"
    
    which we were not detecting as an error condition.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>