Log Message:
Update xen 4.8 packages to 4.8.3. Changes since 4.8.2: include patches from
all security advisory up to and including XSA254.

While there pass XEN_VENDORVERSION=nb${PKGREVISION} to make so that
'xl info' shows the NetBSD PKGREVISION. If PKGREVISION is not available,
define this as 'nb0'.