Revert "virtio-console: remove unnecessary kmemdup()"

This reverts commit 9db81eca.

A dependant patch on this one needs to be reverted, so this one also
needs to be reverted at this point in time.

Link: https://lore.kernel.org/r/208f7a41-a9fa-630c-cb44-c37c503f3a72@kernel.org


Reported-by: Jiri Slaby <jirislaby@kernel.org>
Cc: Xianting Tian <xianting.tian@linux.alibaba.com>
Cc: Shile Zhang <shile.zhang@linux.alibaba.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>