i386/pc: factor out cxl range end to helper

Move calculation of CXL memory region end to separate helper.

This is in preparation to a future change that removes CXL range
dependency on the CXL memory region, with the goal of allowing
pc_pci_hole64_start() to be called before any memory region are
initialized.

Cc: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Acked-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20220719170014.27028-6-joao.m.martins@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>