mmu: do not destroy pt roots with double initialization

the class construction of the page_table_root must happen
before priority "mempool", or all the work in arch-setup
will be destroyed by the class constructor.
Problem noticed while working on the page fault handler for AArch64.

Signed-off-by: Jani Kokkonen <jani.kokkonen@huawei.com>
Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>