virttest.qemu_qtree: Update/improve qemu block related functions

Hi guys,

this pull request contains of few fixes related to qemu monitor "info block" situation. It adds new command to `virttest.qemu_monitor.Monitor` `info_block`, which extracts the data of monitor output and parse them into unified output - dict of dict of each disk params. It supports QMP, old qemu and new (1.5) qemu format and it should in the future supersede the `Monitor.info('block')` calls.

During developing this I fixed couple of bugs, which are also included in this pull request. You can see the details in the individual commits.

Kind regards,
Lukáš