x86/HVM: fold both instances of looking up a hvm_ioreq_vcpu with a request pending

x86/HVM: fold both instances of looking up a hvm_ioreq_vcpu with a request pending

It seems pretty likely that the "break" in the loop getting replaced in
handle_hvm_io_completion() was meant to exit both nested loops at the
same time. Re-purpose what has been hvm_io_pending() to hand back the
struct hvm_ioreq_vcpu instance found, and use it to replace the two
nested loops.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>