scsi-generic: Handle queue full

The sg driver currently has a hardcoded limit of commands it
can handle simultaneously. When this limit is reached the
driver will return -EDOM. So we need to capture this to
enable proper return values here.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
