Fetch remote log files from LLGS tests

Summary:
this enables download of remote log files for llgs and debugserver tests (previously we were just
passing the host file name which obviously did not work). Note this also changes the debugserver
logging to work only when logging has been requested on the command line, whereas previously it
would log unconditionally. I can change it back if anyone is relying on this, but I thought I'd
make this consistent.

Reviewers: tfiala

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D17798

llvm-svn: 262597