Did some testing with your patches:

Test 1:

Xen-Version: Compiled a xen with the 077 commit, your first patch and your
3. (latest) patch for testing.
Passed-through devices: only vga
Result: At some late point in the boot of the domU, the dom0 (the server
itself) restarted itself. This is repeatable. I added the qemu-log and what
I was able to capture from the xl dmesg (I only hit 'xl dmesg' numerous
times until the server restarted and might not have gotten the end of it,
but since there is no xl dmesg log i'm aware off, I couldn't think of
another way. If you have any, please tell me). From the qemu log  I would
guess the restart was caused by some problem in the vram mapping?

Test 2:

Xen-Version: Compiled a normal 077 commit version without any of your
patches
Xen cmd line: added immo=no-sharept as suggested
Passed-through devices: vga, 2 usb controller (ohci+ehci), sound card
Result: works fine. domU boots, no BSOD, no other problems, server seems to
be stable. So not sharing the VT-d page tables seems to do the trick / the
problem is in the page table sharing.