qemu-char: initialize chr_write_lock

Otherwise, Windows fails with a deadlock.

Reported-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1403679897-11480-1-git-send-email-pbonzini@redhat.com
Tested-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
