Calling dma_bdrv_unmap() twice is not necessary and may cause
potential problems if some code changes.
Signed-off-by: NJules Wang <junqing.wang@cs2c.com.cn>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>