examples/power: fix build with some disabled PMDs

examples/power: fix build with some disabled PMDs

Running the devtools/test-build.sh script on IBM Power systems fails
because the IXGBE_PMD is explicity disabled for Power as an untested
driver, but the examples/vm_power_manager application has a hard
dependency on a function call in the IXGBE_PMD.

Modify the example application so that all dependencies on PMD code
are conditionally compiled.

Bugzilla ID: 237
Fixes: c9a4779135c9 ("examples/vm_power_mgr: set MAC address of VF")

Signed-off-by: David Christensen <drc@linux.vnet.ibm.com>
Acked-by: David Hunt <david.hunt@intel.com>