OvmfPkg: rename QemuBootOrderNNNN to VMMBootOrderNNNN

OvmfPkg: rename QemuBootOrderNNNN to VMMBootOrderNNNN

While the actual implementation (using qemu fw_cfg) is qemu-specific,
the idea to store the boot order as configured by the VMM in EFI
variables is not.  So lets give the variables a more neutral name while
we still can (i.e. no stable tag yet with the new feature).

While being at it also fix the NNNN format (use %x instead of %d for
consistency with BootNNNN).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>