hw/xen: Use g_free instead of free and fix potential memory leaks

The wrong functions and the missing calls of g_free were reported
by cppcheck.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
