char: drop data written to a disconnected pty

When a serial port writes data to a pty that's disconnected, drop the
data and return the length dropped. This avoids triggering pointless
retries in callers like the 16550A serial_xmit(), and causes
qemu_chr_fe_write() to write all data to the log file, rather than
logging only while a pty client like virsh console happens to be
connected.

Signed-off-by: Ed Swierk <eswierk@skyportsystems.com>
Message-Id: <1485870329-79428-1-git-send-email-eswierk@skyportsystems.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>