As Manuel would say, "root=/dev/hda1" is a copy and paste from a
linux config.
The correct line would be "bootdev=wd0", obviously replacing wd0
with your
applicable nbsd device.
I believe root= is now an equivelent? ... but you would still
need to specify
a nbsd device, not a linux one.
The NetBSD/xen howto still uses the "root=/dev/hda0" terminology, so
it is not so much a copy-and-paste from Linux as a copy-and-paste
from the NetBSD instructions.
I use "root=/dev/hda0" all over my dom0's and, no, I do not have a
"hda0" entry in my /dev.