fix just-broken "virsh start" and "virsh pool-start" commands

fix just-broken "virsh start" and "virsh pool-start" commands

* src/virsh.c (cmdPoolStart, cmdStart): Change hard-coded
vshCommandOptDomainBy string argument to match just-changed
option name.  Cole Robinson reported that "virsh start" was
broken and provided that part of the fix.
Bug introduced by yesterday's "virsh.c: tweak options to produce
more accurate help".
* tests/start: New file.  Test for the above fix.
* tests/Makefile.am (test_scripts): Add start.