[SCSI] virtio_scsi: fix memory leak on full queue condition.

virtscsi_queuecommand was leaking memory when the virtio queue was full.

Tested: Guest operates correctly even with very small queue sizes, validated
we're not leaking kmalloc-192 sized allocations anymore.

Signed-off-by: Eric Northup <digitaleric@google.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>