Add virsh desc testing, include virsh cmd update, new test and new config

This case include following parts:
1). For running domain, get/set description&title with options.
2). For shut off domian, get/set description&title with options.
3). For persistent/transient domain, get/set description&title with options.
4). Several negative test

Signed-off-by: Yanbing Du <ydu@redhat.com>
Fix several problems in PR#786

1). virsh.desc function lose a parameter to set 'desc message'
2). Use vm.is_dead/alive() instead vm.state(), which make the code more clear
3). Fix the repeat params get of "main_vm"

Signed-off-by: Yanbing Du <ydu@redhat.com>