xen/console: fix priority of Xen console

Currently the Xen console is always attached with priority CN_REMOTE
(highest), which means that when booting with a single console the Xen
console will take preference over the VGA for example, and that's not
intended unless the user has also selected to use a serial console.

Fix this by lowering the priority of the Xen console to NORMAL unless
the user has selected to use a serial console. This keeps the usual
FreeBSD behavior of outputting to the internal consoles (ie: VGA) when
booted as a Xen dom0.

MFC after: 3 days
Sponsored by: Citrix Systems R&D