ArmVirtPkg/PlatformBootManagerLib: Use EDKII_SERIAL_PORT_LIB_VENDOR_GUID

ArmVirtPkg/PlatformBootManagerLib: Use EDKII_SERIAL_PORT_LIB_VENDOR_GUID

SERIAL_DXE_FILE_GUID is now defined in MdeModulePkg as
EDKII_SERIAL_PORT_LIB_VENDOR_GUID, simply use it.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20190606131459.1464-3-anthony.perard@citrix.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
[lersek@redhat.com: drop "from MdeModulePkg" from the subject]
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>