vpci: fix deferral of long operations

vpci: fix deferral of long operations

Current logic to handle long running operations is flawed because it
doesn't prevent the guest vcpu from running. Fix this by raising a
scheduler softirq when preemption is required, so that the do_softirq
call in the guest entry path performs a rescheduling. Also move the
call to vpci_process_pending into handle_hvm_io_completion, together
with the IOREQ code that handles pending IO instructions.

Note that a scheduler softirq is also raised when the long running
operation is queued in order to prevent the guest vcpu from resuming
execution.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>