BUG: FreeBSD fails to boot on xen-unstable tip

["Roger Pau Monné" <roger.pau@citrix.com>]

Hello,

FreeBSD bootloader is getting stuck with tip of xen-unstable. I've
bisected the Xen tree and found that the culprit of this is the
following commit:

527824f41f5fac9cba3d4441b2e73d3118d98837 is the first bad commit
commit 527824f41f5fac9cba3d4441b2e73d3118d98837
Author: Tim Deegan <tim@xen.org>
Date:   Thu Mar 28 11:27:31 2013 +0000

    x86/hvm: Centralize and simplify the RTC IRQ logic.

    This keeps the behaviour of strobing the IRQ line every time any RTC
    interrupt source is raised.  I rather suspect (based on the behaviour
    of the MC146818A RTC) that we ought to be suppressing all subsequent
    interrupts whenever RTC_IRQF is set, but this way avoids making
    guest-visible changes.

    Signed-off-by: Tim Deegan <tim@xen.org>
    Acked-by: Jan Beulich <jbeulich@suse.com>
    Acked-by: Keir Fraser <keir@xen.org>

Not sure, but I guess this is probably related to the issues George is
seeing on Windows 2003:

<CAFLBxZbaggssHmQFpEkreFDsqdu3XPDqd1k4fXQb9k=qSXjMcw@mail.gmail.com>

Roger.