bs_sheepdog.c: fix error handling issue in bs_sheepdog_request()

bs_sheepdog_request function, the request handler of sheepdog driver,
doesn't set scsi status code if the driver failed to create writable
VDI from snapshot.

In that case, CHECK_CONDITION result code should be set to cmd->result
through scsi_set_result() macro.  This fix corrects the error
handling issue.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>