Our virDomainBlockStatsFlags API uses the old approach where, when it's
called without the typed parameter array, returns the count of parameters
supported by qemu.

The supported parameter count is obtained via separate monitor calls
which is a waste since we can calculate it when gathering the data.

This patch adds code to the qemuMonitorGetAllBlockStatsInfo workers that
allows to track the count of supported fields reported by qemu and will
allow to remove the old duplicate code.