xen/swiotlb: Move the error strings to its own function.

That way we can more easily reuse those errors when using the
late SWIOTLB init.

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