block: Fix bs->request_alignment assertion for bs->sg=1

For sg backends, bs->request_alignment is meaningless and may be 0.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>