virttest.qemu_monitor: Parse "info block" into dictionary

As there is more and more versions of "info block" this patch
parses the output and creates dictionary of each disk params
(dict of dicts).

Currently supported methods are QMP, HMP old qemu and HMP qemu 1.5.
QMP representation contains the most informations, HMP qemu 1.5 the
least. The missing informations should have default value and it's
upto the user to set it. (use dict.get(key, default))

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
virttest.qemu_qtree: Use Monitor.info_block() to get block info

Use Monitor.info_block() which covers all versions of "info block"
command to get the information.

Also update the cases which use this function to provide info_block
output instead the Monitor.info('block') one.

Last but not least add unittest.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>