x86: realmode: fix serial_init()

In some setups serial output from the real-mode tests is corrupted.

I do not know the serial port initialization code well, but the
protected mode initialization code is different than the real-mode code.
Using the protected mode serial port initialization fixes the problem.

Keeping the tradition of code duplication between real-mode and
protected mode, this patch copies the missing initialization into
real-mode serial port initialization.

Signed-off-by: Nadav Amit <namit@vmware.com>
Message-Id: <20200701193045.31247-1-namit@vmware.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>