xen/arm: process pending vPCI map/unmap operations

xen/arm: process pending vPCI map/unmap operations

vPCI may map and unmap PCI device memory (BARs) being passed through which
may take a lot of time. For this those operations may be deferred to be
performed later, so that they can be safely preempted.

Currently this deferred processing is happening in common IOREQ code
which doesn't seem to be the right place for x86 and is even more
doubtful because IOREQ may not be enabled for Arm at all.
So, for Arm the pending vPCI work may have no chance to be executed
if the processing is left as is in the common IOREQ code only.
For that reason make vPCI processing happen in arch specific code.

Please be aware that there are a few outstanding TODOs affecting this
code path, see xen/drivers/vpci/header.c:map_range and
xen/drivers/vpci/header.c:vpci_process_pending.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
[x86 part]
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Rahul Singh <rahul.singh@arm.com>
Tested-by: Rahul Singh <rahul.singh@arm.com>