i386/pc: factor out cxl range start to helper

Factor out the calculation of the base address of the memory region.
It will be used later on for the cxl range end counterpart calculation
and as well in pc_memory_init() CXL memory region initialization, thus
avoiding duplication.

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: <20220719170014.27028-7-joao.m.martins@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>