qemu-char: correct return value from chr_read functions

Even if a CharDriverState's source is blocked by the front-end,
it must not be dropped. The IOWatchPoll that wraps it will take
care of adding and removing it to the main loop.  Only remove
the source when the channel is closed; and in that case, make sure
that the wrapping IOWatchPoll is removed too.

These should just be theoretical bugs.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1366385529-10329-4-git-send-email-pbonzini@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
