efi_loader: support booting semihosting file

Executing an EFI binary fails for files loaded via semihosting.

Construct a dummy device path for EFI binaries loaded via semihosting.

A future complete solution may include the creation of a handle with a
simple file system protocol.

Reported-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>