scsi-disk: Fix crash if underlying host file or disk returns error

scsi-disk: Fix crash if underlying host file or disk returns error

Commit 40dce4ee6 "scsi-disk: fix rerror/werror=ignore" introduced a
bug which causes qemu to crash with the assertion error below if the
host file or disk returns an error:

  qemu-system-x86_64: hw/scsi/scsi-bus.c:1374: scsi_req_complete:
  Assertion `req->status == -1' failed.

Kevin Wolf suggested this fix:

  < kwolf> Hm, should the final return false; in that patch
           actually be a return true?
  < kwolf> Because I think he didn't intend to change anything
           except BLOCK_ERROR_ACTION_IGNORE

Buglink: https://bugs.launchpad.net/qemu/+bug/1804323
Fixes: 40dce4ee61c68395f6d463fae792f61b7c003bce
Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>