Fix path generation for Dhcpv4 bootloader.

Right now we always look for e.g. "\grubx64.efi", which is completely
wrong.  This makes it look for the path shim was loaded from and modify
that to end in a sanitized version of our default loader name.

Resolves: rhbz#1032583

Signed-off-by: Peter Jones <pjones@redhat.com>
Lengths that might be -1 can't be unsigned, Peter.

Signed-off-by: Peter Jones <pjones@redhat.com>