xen: Add missing va_end() in hypercall_create_continuation()

xen: Add missing va_end() in hypercall_create_continuation()

The documentation requires va_start() to always be matched with a
corresponding va_end(). However, this is not the case in the path used
for bad format.

This was introduced by XSA-296.

Coverity-ID: 1488727
Fixes: 0bf9f8d3e3 ("xen/hypercall: Don't use BUG() for parameter checking in hypercall_create_continuation()")
Signed-off-by: Julien Grall <julien@xen.org>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>