chardev: fix parallel device can't be reconnect

chardev: fix parallel device can't be reconnect

Parallel device don't register be->chr_can_read function, but remote
disconnect event is handled in chr_read.So connected parallel device
can not detect remote disconnect event. The chardevs with chr_can_read=NULL
has the same problem.

Signed-off-by: Peng Hao <[email protected]>
Reviewed-by: Wang Yechao <[email protected]>
Reviewed-by: Jiang Biao <[email protected]>
Message-Id: <1499874119[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>