Xen: use qemu_strtoul instead of strtol

No need to roll our own (with slightly incorrect handling of errno),
when we can use the common version.

Change signed parsing to unsigned, because what it read are values in
PCI config space, which are non-negative.

Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
