In case efi runtime disabled via noefi kernel cmdline
arm64_enter_virtual_mode should error out.

At the same time move early_memunmap(memmap.map, mapsize) to the
beginning of the function or it will leak early mem.
Signed-off-by: NDave Young <dyoung@redhat.com>
Reviewed-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NMatt Fleming <matt.fleming@intel.com>