examples/power: support meson/ninja build

examples/power: support meson/ninja build

Add meson.build in vm_power_manager and the guest_cli subdirectory.
Building can be achieved by going to the build directory, and using

meson configure -Dexamples=vm_power_manager,vm_power_manager/guest_cli

Then, when ninja is invoked, it will build dpdk-vm_power_manger and
dpdk-guest_cli

Work still needs to be done on the meson build system to handles the case
where the target list of example apps is defined as 'all'. That will come
in a future patch.

Signed-off-by: David Hunt <david.hunt@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>