scsi: allow user to set werror as report

'enospc' is the default for -drive, but qemu allows user to set
drive option werror. If werror of scsi-generic is set to 'report'
by user, qemu will not allow vm to start.

This patch allow user to set werror as 'report' for scsi-generic.

Signed-off-by: Zihao Chang <changzihao1@huawei.com>
Reviewed-by: Fam Zheng <fam@euphon.net>
Message-Id: <20201103061240.1364-1-changzihao1@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>