usb-host: handle guest-issued clear halt

usb-host: handle guest-issued clear halt

Most important here is to update our internal endpoint state so we know
the endpoint isn't in halted state any more.  Without this usb-host
tries to clear halt again with the next data transfer submitted.  Doing
this twice is (a) not correct and (b) confuses some usb devices,
rendering them non-functional in the guest.

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