xen/arm: assign static shared memory to the default owner dom_io

xen/arm: assign static shared memory to the default owner dom_io

This commit introduces process_shm to cope with static shared memory in
domain construction.

DOMID_IO will be the default owner of memory pre-shared among multiple domains
at boot time, when no explicit owner is specified.
And DOMID_IO is a fake domain and is not described in the Device-Tree.
Therefore When the owner of the shared region is DOMID_IO, we will only
find the borrowers when parsing the Device-Tree.
When we found the first borrower of the region, we need to assign the region to
DOMID_IO

This commit only considers allocating static shared memory to dom_io
when owner domain is not explicitly defined in device tree, all the left,
including the "borrower" code path, the "explicit owner" code path, shall
be introduced later in the following patches.

Signed-off-by: Penny Zheng <penny.zheng@arm.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>