qemu-char: make writes thread-safe

This will let threads other than the I/O thread raise QMP events.

GIOChannel is thread-safe, and send and receive state is usually
well-separated.  The only driver that requires some care is the
pty driver, where some of the state is shared by the read and write
sides.  That state is protected with the chr_write_lock too.

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>