xen: Automatically find serial port on PCI/PCIe and AMT devices.

xen: Automatically find serial port on PCI/PCIe and AMT devices.

Instead of having to manually look the right I/O port on the PCI
devices or AMT devices, lets probe the card and find that
automatically.

This means that you don't have to have this:
 com1=115200,8n1,0xd800,0

But instead can have
 com1=115200,8n1,magic

Or if you have AMT:
 com1=19200,8n1,amt

Signed-off-by: James Mckenzie <jamesmck@bob.uk.xensource.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Tom Goetz <tom.goetz@virtualcomputer.com>