xen/sysctl: fix XEN_SYSCTL_getdomaininfolist handling with XSM

xen/sysctl: fix XEN_SYSCTL_getdomaininfolist handling with XSM

In case XSM is active, the handling of XEN_SYSCTL_getdomaininfolist
can fail if the last domain scanned isn't allowed to be accessed by
the calling domain (i.e. xsm_getdomaininfo(XSM_HOOK, d) is failing).

Fix that by just ignoring scanned domains where xsm_getdomaininfo()
is returning an error, like it is effectively done when such a
situation occurs for a domain not being the last one scanned.

Fixes: d046f361dc93 ("Xen Security Modules: XSM")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>