Bluetooth: Add support for vectored writes to virtual HCI driver

The Bluetooth virtual HCI driver is using a misc character device to
allow emulation of HCI devices from userspace. This change enables the
support for vectored writes. Previously this was failing with EINVAL
since no complete H:4 packet was written.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>