Various minor bootperf improvements
Various minor bootperf improvements

Change-Id: I791fa01fcd2feb0c7b4c7e3d63cebc88d896c8ac

BUG=chromium-os:9459
BUG=chromium-os:9460
TEST=run the script, and check results with showbootdata or by manually inspecting the output directory
TEST=run "bootperf invalid-hostname", and see that the error message is useful

Review URL: http://codereview.chromium.org/5156008

endocode/scripts,bcwaldon/coreos-scripts,endocode/scripts,coreos/scripts,endocode/scripts,smilart/scripts,vmware/coreos-scripts,mjg59/scripts,fivethreeo/scripts,mischief/scripts,nicescale/scripts,andrejro/coreos--scripts,andrejro/coreos--scripts,crawford/scripts,marineam/coreos-scripts,BugRoger/scripts,mjg59/scripts,smilart/scripts,andrejro/coreos--scripts,zhang0137/scripts,smilart/scripts,gtank/scripts,crawford/scripts,gtank/scripts,trnubo/scripts,fivethreeo/scripts,marineam/coreos-scripts,nicescale/scripts,BugRoger/scripts,nicescale/scripts,fivethreeo/scripts,mischief/scripts,glevand/coreos--scripts,gtank/scripts,vmware/coreos-scripts,bcwaldon/coreos-scripts,coreos/scripts,mjg59/scripts,glevand/coreos--scripts,crawford/scripts,bcwaldon/coreos-scripts,sigma/coreos-scripts,vmware/coreos-scripts,trnubo/scripts,zhang0137/scripts,trnubo/scripts,sigma/coreos-scripts,sigma/coreos-scripts,zhang0137/scripts,mischief/scripts,marineam/coreos-scripts,BugRoger/scripts