usb-redir: Don't delay handling of open events to a bottom half

usb-redir: Don't delay handling of open events to a bottom half

There is no need for this, and doing so means that a backend trying to
write immediately after an open event will see qemu_chr_be_can_write
returning 0, which not all backends handle well as there is no wakeup
mechanism to detect when the frontend does become writable.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>