efi_loader: Allocate memory handle for mem dp

efi_loader: Allocate memory handle for mem dp

When we boot using memdp (bootefi on an address without previous
load that populates the device path) then the memory device path
we pass in is not backed by any handle.

That can result in weird effects. For example grub gets very grumpy
about this inside the efi_net module and just loops endlessly.

So let's expose a simple handle that the memory device path is backed
on. That way any code that looks for the device the dp is on, finds
one.

Signed-off-by: Alexander Graf <agraf@suse.de>