xhci: tweak limits

Set maxports to 15.  This is what the usb3 route string can handle.

Set maxslots to 64.  This is more than the number of root ports we
can have, but with additional hubs you can end up with more devices.

Set maxintrs (aka msi vectors) to 16.  Should be enougth, especially
considering that vectors are a limited ressource.  Linux guests use
only three at the moment.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
