Fix the efi serial console in the Arm models

[andrew]

Summary

On some UEFI implementations the ConsOut EFI variable is not a device
path end type so we never move to the next node. Fix this by always
incrementing the device path node pointer, with a sanity check that
the node length is large enough so no two nodes overlap.

While here retorn failure on malloc failure rather than a NULL pointer
dereference.