Now that qemuDomainBlocksStatsGather provides functions of both
qemuMonitorGetBlockStatsParamsNumber and qemuMonitorGetBlockStatsInfo we
can reuse it and kill a lot of code.

Additionally as a bonus qemuDomainBlockStatsFlags will now support
summary statistics so add a statement to the virsh man page about that.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1142636