nbd: Return correct error for write to read-only export

The NBD Protocol requires that servers should send EPERM for
attempts to write (or trim) a read-only export.  We were
correct for TRIM (blk_co_discard() gave EPERM); but were
manually setting EROFS which then got mapped to EINVAL over
the wire on writes.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <1459913704-19949-2-git-send-email-eblake@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

nbd/server.c

@@ -26,6 +26,7 @@ static int system_errno_to_nbd_errno(int err)
     case 0:
         return NBD_SUCCESS;
     case EPERM:
+    case EROFS:
         return NBD_EPERM;
     case EIO:
         return NBD_EIO;