hw/9pfs: Fix unchecked strdup() by converting to g_strdup()

Note: the allocation in virtio_9p_init() is still leaked.  To be fixed
in a followup commit.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
