x86: io: force carriage return on \n put to serial port

When '\n' (line feed) is put to serial port, put '\r' (carriage return) as
well. This makes the serial port output to look properly (without stairs) at
real hardware. Also the output looks well at QEMU/KVM and BOCHS emulators.

Signed-off-by: Eugene Korenevsky <ekorenevsky@gmail.com>
Message-Id: <20180721185915.GA21451@vnote>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>