xhci: emulate intr endpoint intervals correctly

Respect the interval for interrupt endpoints, so we don't finish
transfers as fast as possible but at the rate configured by the guest.

Fixes guest deadlocks triggered by interrupt storms.

Cc:
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit 4d7a81c0)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>