xen/swiotlb: Move the nr_tbl determination in its own function.

Moving the function out of the way to prepare for the late
SWIOTLB init.

Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>