libxc: add MMIO hole size parameter to xc_hvm_build()

libxc: add MMIO hole size parameter to xc_hvm_build()

Add a parameter for the MMIO hole size when building a HVM domain.

This is useful for specifying a larger than normal MMIO hole to ensure
that no PCIe device's MMIO region overlaps with RAM in a guest's
physical address space.  This is needed on certain systems with PCIe
switches with a broken ACS capability.  On these systems, if a device
downstream of the switch attempts a DMA to a guest physical address
that overlaps with the MMIO region of another downstream device, then
the switch routes the transfer directly to the device and the read or
write never hits RAM.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>