x86/hvm/emulate: make sure rep I/O emulation does not cross GFN boundaries

x86/hvm/emulate: make sure rep I/O emulation does not cross GFN boundaries

When emulating a rep I/O operation it is possible that the ioreq will
describe a single operation that spans multiple GFNs. This is fine as long
as all those GFNs fall within an MMIO region covered by a single device
model, but unfortunately the higher levels of the emulation code do not
guarantee that. This is something that should almost certainly be fixed,
but in the meantime this patch makes sure that MMIO is truncated at GFN
boundaries and hence the appropriate device model is re-evaluated for each
target GFN.

NOTE: This patch does not deal with the case of a single MMIO operation
      spanning a GFN boundary. That is more complex to deal with and is
      deferred to a subsequent patch.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Convert calculations to be 32-bit only.

Signed-off-by: Jan Beulich <jbeulich@suse.com>