Some devices seem to choke on receiving a USB_REQ_GET_CONFIGURATION ctrl msg
(witnessed with a digital picture frame usb id 1908:1320).
When usb_fs_type == USB_FS_SYS, the active configuration can be read directly
from sysfs, which allows using this device through qemu's usb redirection.
More in general it seems a good idea to not send needless control msg's to
devices, esp. as the code in question is called every time a set_interface
is done. Which happens multiple times during virtual machine startup, and
when device drivers are activating the usb device.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>