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>
general: PEP8 fixes

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>