MdeModulePkg: Introduce EDKII_SERIAL_PORT_LIB_VENDOR_GUID

MdeModulePkg: Introduce EDKII_SERIAL_PORT_LIB_VENDOR_GUID

SERIAL_DXE_FILE_GUID is used in different places, create a single
define that other can use. Also fix SerialDxe to actually use
EDKII_SERIAL_PORT_LIB_VENDOR_GUID in the device path instead of the
FILE_GUID.

Suggested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20190606131459.1464-2-anthony.perard@citrix.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>