This fixes several problems I found in the UART serial implementation.
Now all divisor values are allowed, while before divisor values of zero
and below the base baud rate were rejected. All changes are in reference
to http://www.sci.muni.cz/docs/pc/serport.txt



Signed-off-by: Calvin Lee <cyrus296@gmail.com>
Message-Id: <20180512000545.966-2-cyrus296@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>