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 4d7a81c06f5f17e019a2d3a18300500bd64f6f40)

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