arm64/efi: Do not enter virtual mode if booting with efi=noruntime or noefi

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: Dave Young <dyoung@redhat.com>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>