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>