Important note for HVM guests backed with LVM

[Roy Marples <roy%marples.name@localhost>]

Hi List!

My XEN NetBSD DomU has been chugging along really well and I'm very 
pleased with it so far.

So I tried to get FreeBSD and OpenBSD HVM guests running (as they don't 
have PV support like we do).
However, all XEN documentation (theirs, ours) is utterly useless for 
trying to get this to work with LVM backing.
Errors are FreeBSD installing fine, but is then presented with an 
unbootable disk.
OpenBSD just reports errors talking to wd0 and refuses to install.
If you're quick enough to attach a VNC console to the guest you can see 
QEMU reporting a disk size of 0.

Looking at the qemu log in the Dom0, I see this:
Using file /dev/vg0/freebsd in read-write mode
qemu: could not open vbd '/local/domain/0/backend/vbd/15/768/mode' or 
hard disk image '/dev/vg0/freebsd' (drv 'phy' format 'raw')

And here's my config:
disk = [
        'phy:/dev/vg0/freebsd,xvda,w',
        
'file:/home/roy/FreeBSD-11.0-RELEASE-amd64-disc1.iso,xvdc:cdrom,r'
]

That matches every piece of documentation I could find.
THIS IS WRONG!!!!

Then, by a sheer fluke I found this thread:
http://port-xen.netbsd.narkive.com/MF6vPA3P/xen-and-lvm-backed-domu-s
https://mail-index.netbsd.org/port-xen/2010/03/12/msg005804.html

I said it worked fine, but note the slight difference in stating the 
device.
So I changed my config to this:

disk = [
        'phy:/dev/mapper/vg0-freebsd,xvda,w',
        
'file:/home/roy/FreeBSD-11.0-RELEASE-amd64-disc1.iso,xvdc:cdrom,r'
]

And OMG it works fine!
So can someone please add this to our howto? I think it's quite 
important.

BTW, does anyone know what the difference is between xvda and ioemu:hda 
is? The former is more descriptive to me, but I cannot find if that 
affects anything or not "recommended".

Roy