This is a branch of bug#966449.  I'm copying in the pertinent text.

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I have also been testing leap with xen - two platforms:

a) HP Proliant DL360G5, quad-core, 8Gb, dual-GigE, single SAS drive.
b) Fujitsu Desktop, dual-core, 4Gb, GigE, single SATA drive.

On (a) it went fine, no problems whatsoever.  On (b) I did eventually manage to
get an installation to work, but it's essentially unusable.  During both
startup and shutdown, it hangs several times causing both to take forever. Only
man
ual intervention (pressing enter) at the console gets the process to continue.

I have no idea what is happening, but have seen messages such as :

rcu_sched kthread starved for N jiffies!
rcu_sched self-detected stall on CPU ...

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(In reply to Charles Arnold from comment #9)
> Please download an install the following RPMs for your os42.1 host:
>
> xen-4.5.2_06-5.1.x86_64.rpm
> xen-libs-4.5.2_06-5.1.x86_64.rpm
> xen-tools-4.5.2_06-5.1.x86_64.rpm
>
> from this location,
>
> http://download.opensuse.org/repositories/Virtualization:/openSUSE42.1/
> openSUSE_Leap_42.1/x86_64/
>
> Once installed set the following hypervisor boot flag.
>
>   Edit /etc/default/grub and add,
>     GRUB_CMDLINE_XEN_DEFAULT="iommu=no-igfx"
>   and then run,
>     grub2-mkconfig -o /boot/grub2/grub.cfg
>
> Reboot the host and select the xen menu entry. Please report your results.

On my test system (Fujitsu desktop), this was a significant improvement. 
During boot-up, I only needed to press enter once or twice, but during shutdown
I still needed to hit enter several times to complete.

-------------------------
(In reply to Charles Arnold from comment #12)
> (In reply to Per Jessen from comment #11)
> > (In reply to Per Jessen from comment #10)
> > > On my test system (Fujitsu desktop), this was a significant improvement.
> > > During boot-up, I only needed to press enter once or twice, and once 
during shutdown.
> >
> > Uh, correction - it did require hitting enter several times to complete a
> > shutdown/reboot.
>
> Could you explain what you are seeing that requires hitting the Enter
> key several times?

"requires" is perhaps the wrong word. I'm at the physical box with a console,
and during boot-up or shutdown, it appears to just halt every so often. Hitting
enter makes it continue.  Then it halts, I hit enter etc.  Without hitting
enter, boot-up or shutdown never finishes.