Bad continuation handling in GNTTABOP_copy

ISSUE DESCRIPTION

Grant table operations are expected to return 0 for success, and a negative number for errors. The fix for CVE-2017-12135 / XSA-226 introduced a path through grant copy handling where success may be returned to the caller without any action taken.
In particular the status fields of individual operations are left uninitialised, and may result in errant behaviour in the caller of GNTTABOP_copy.

IMPACT

A buggy or malicious guest can construct its grant table in such a way that, when a backend domain tries to copy a grant, it hits the incorrect exit path.
This returns success to the caller without doing anything, which may cause in crashes or other incorrect behaviour.

VULNERABLE SYSTEMS

Systems running any version of Xen are vulnerable.