efi_loader: check device path in InstallMultipleProtocolInterfaces

According to the UEFI spec InstallMultipleProtocolInterfaces() must check
if a device path has already been installed. In this case it must return
EFI_ALREADY_STARTED.

Cf. UEFI SCT II 2.6 A (2017),
3.3.16 InstallMultipleProtocolInterfaces(), 5.1.3.16.1.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>