ehci: apply limit to iTD/sidt descriptors

ehci: apply limit to iTD/sidt descriptors

Commit "156a2e4 ehci: make idt processing more robust" tries to avoid a
DoS by the guest (create a circular iTD queue and let qemu ehci
emulation run in circles forever).  Unfortunately this has two problems:
First it misses the case of siTDs, and second it reportedly breaks
FreeBSD.

So lets go for a different approach: just count the number of iTDs and
siTDs we have seen per frame and apply a limit.  That should really
catch all cases now.

Reported-by: 杜少博 <dushaobo@360.cn>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit 1ae3f2f178087711f9591350abad133525ba93f2)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>