dma-helpers: avoid calling dma_bdrv_unmap() twice

Calling dma_bdrv_unmap() twice is not necessary and may cause
potential problems if some code changes.

Signed-off-by: Jules Wang <junqing.wang@cs2c.com.cn>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>