qemu-ga: isa-serial support on Windows

Add support for isa-serial method for qemu-ga on Windows,
Added -p command line parameter for serial port name
specification, e.g. "-p COM15".

Signed-off-by: Miki Mishael <mmishael@redhat.com>
Signed-off-by: Dmitry Fleytman <dfleytma@redhat.com>

*added default isa-serial path to help output

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>