xen: x86: copy back tsc info, not pointer to tsc info in domctl

xen: x86: copy back tsc info, not pointer to tsc info in domctl

In 38b37ed82705 "x86/domctl: cleanup", XEN_DOMCTL_gettscinfo was
changed to use the standard copyback mechanism.

However the output TSC Info is a guerst handle, i.e. a pointer to the
location for the information, copyback just copies the unchanged
pointer back.

Switch back to fetching the details into a local struct and explicitly
copying it back.

This caused test failures in the Cambridge instance of osstest, but
not for some reason in the production instance.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>