Arm/domctl: correct XEN_DOMCTL_vuart_op error return value

copy_to_guest() returns the number of bytes not copied; that's not what
the function should return to its caller though. Convert to returning
-EFAULT instead.

Fixes: 86039f2e8c ("xen/arm: vpl011: Add a new domctl API to initialize vpl011")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Michal Orzel <michal.orzel@amd.com>

xen/arch/arm/domctl.c

@@ -172,8 +172,8 @@ long arch_do_domctl(struct xen_domctl *domctl, struct domain *d,
             break;
         }
 
-        if ( !rc )
-            rc = copy_to_guest(u_domctl, domctl, 1);
+        if ( !rc && copy_to_guest(u_domctl, domctl, 1) )
+            rc = -EFAULT;
 
         return rc;
     }