OvmfPkg/PlatformBootManagerLib: Use a Xen console for ConOut/ConIn

OvmfPkg/PlatformBootManagerLib: Use a Xen console for ConOut/ConIn

On a Xen PVH guest, none of the existing serial or console interface
works, so we add a new one, based on XenConsoleSerialPortLib, and
implemented via SerialDxe.

That is a simple console implementation that can work on both PVH
guest and HVM guests, even if it is rarely going to be used on HVM.

Have PlatformBootManagerLib look for the new console, when running as a
Xen guest.

Since we use VENDOR_UART_DEVICE_PATH, fix its description and coding
style.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1689
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20190813113119.14804-32-anthony.perard@citrix.com>