xen/arm: Add additional reference to owner domain when the owner is allocated

xen/arm: Add additional reference to owner domain when the owner is allocated

Borrower domain will fail to get a page ref using the owner domain
during allocation, when the owner is created after borrower.

So here, we decide to get and add the right amount of reference, which
is the number of borrowers, when the owner is allocated.

Signed-off-by: Penny Zheng <penny.zheng@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Julien Grall <jgrall@amazon.com>