qemu-char: use consistent idiom for removing sources

Always check that the source is active, and zero the tag afterwards.

The occurrence in pty_chr_state will trigger with the next patch, the
others are just theoretical.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1366385529-10329-2-git-send-email-pbonzini@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
