Can create PV domain in 4.2 only with xm and not xl

[Andi Reinbrech <andi@darwinistic.com>]

Hi Xenners,

This problem has been cropping up since late 4.2-unstable builds:  I can
create all my HVM guests (windows 7, 8, XP and XP64) with xl create, but
my existing PV guests (Fedora based) don't start.  They either hang or
time out with an error that they can't connect to a vbd backend device.

This happens after the GRUB menu, so the bootloader is in fact accessed.

The strangest thing is, that by starting xend and creating the same
domain with xm create works 100%, but after stopping xend afterwards I
cannot create any more domains with xl, because udev gets messed up
between xm and xl.  Obviously I shouldn't, and don't want to mix the
two.

Another strange thing is that xl create on this same domain works fine
on kernel 3.5...  But I can't use kernel >=3.5, as the PCI USB
passthrough is broken.

So I am sitting with a catch-22: use kernel 3.5/3.6 and have xl work for
all domains, but can't use my HVM Windows machine that needs PCI
passthrough for VGA and USB, or use only kernel 3.3.4, but can't start
PV domains unless I use xend.

Any suggestions would be appreciated!  If I need to send configs or
logs, please let me know.

Kind regards,
Andi