OvmfPkg: Update code to be more C11 compliant by using __func__

__FUNCTION__ is a pre-standard extension that gcc and Visual C++ among
others support, while __func__ was standardized in C99.

Since it's more standard, replace __FUNCTION__ with __func__ throughout
OvmfPkg.

Signed-off-by: Rebecca Cran <rebecca@bsdio.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>

OvmfPkg/Library/XenIoMmioLib/XenIoMmioLib.c

@@ -91,7 +91,7 @@ XenIoMmioInstall (
                        &mXenBusRootDevicePathTemplate
                        );
   if (!XenBusDevicePath) {
-    DEBUG ((DEBUG_ERROR, "%a: Out of memory\n", __FUNCTION__));
+    DEBUG ((DEBUG_ERROR, "%a: Out of memory\n", __func__));
     Status = EFI_OUT_OF_RESOURCES;
     goto FreeXenIo;
   }
@@ -115,7 +115,7 @@ XenIoMmioInstall (
     DEBUG_ERROR,
     "%a: Failed to install the EFI_DEVICE_PATH and "
     "XENIO_PROTOCOL protocols on handle %p (Status == %r)\n",
-    __FUNCTION__,
+    __func__,
     OutHandle,
     Status
     ));