virtio_console: Initialize guest_connected=true for rproc_serial

When rproc_serial is initialized, guest_connected should be set
to true. We can then revert the extra checks introduced in
commit: "virtio_console: Let unconnected rproc device receive data."

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>