OvmfPkg: Create platform OvmfXen

OvmfPkg: Create platform OvmfXen

OvmfXen is a copy of OvmfX64, removing VirtIO and some SMM.

This new platform will be changed to make it works on two types of Xen
guest: HVM and PVH.

Compare to OvmfX64, this patch:

- changed: PLATFORM_GUID, OUTPUT_DIRECTORY, FLASH_DEFINITION
- removed: VirtioLib class resolution
- removed: all UEFI_DRIVER modules for virtio devices
- removed: DXE_SMM_DRIVER and SMM_CORE lib class resolutions
- removed: DXE_SMM_DRIVER and SMM_CORE FDF rules
- removed: Everything related to SMM_REQUIRE==true
- removed: Everything related to SECURE_BOOT_ENABLE==true
- removed: Everything related to TPM2_ENABLE==true
- changed: PcdPciDisableBusEnumeration dynamic default flipped to TRUE
- changed: default FD_SIZE_IN_KB to 2M.
- reverted d272449d9e1e, "OvmfPkg: raise DXEFV size to 11 MB"

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: <20190813113119.14804-3-anthony.perard@citrix.com>