Enable PCI serial devices for console messages

Enable PCI serial devices for console messages

The basic issue is that some PCI serial devices use a non-standard
crystal to control the baud rate divisor.  This patch enhances the
`com' parameter to enable you to specify the crystal frequency used by
the serial device. Since this parameter already allows you to specify
I/O address and IRQ this is all that is needed to get a PCI serial
device to work.  With this patch the `com' parameter is now defined
as:
        comN=BBB[/CCC][,NPS[,III[,IRQ]]]

Where `CCC' is the crystal frequency. Note that if you specify 0 for
`IRQ' Xen will run the serial device in polled mode, obviating the
need for interrupts (an advantage since this is the only way I was
able to get my PCI serial device to work).

As an example, I am using the xen boot command:

        kernel /xen.gz com1=3D115200,8n1,0xe880/921600,0
        console=3Dcom1

Signed-off-by: Don Dugger <donald.d.dugger@intel.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>