doc: add core queries in power example guide

doc: add core queries in power example guide

This patch adds some minor updates for the vm_power_manager and guest_cli
example applications.

Now that the virtio-serial channels between vm_power manager and
the guest_cli has bi-directional capability, there are some new commands.

Firstly, the command in vm_power_manager to enable queries for a given VM:

  * set_query {vm_name} enable|disable

Then, commands to query the frequencies and capabilities of the cores
in the VM:

  * query_cpu_freq {core_num}|all
  * query_cpu_caps {core_num}|all

Signed-off-by: David Hunt <david.hunt@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>