If we ensure that virNodeSuspendGetTargetMask always resets
*bitmask to zero upon failure, there is no need for the
powerMgmt_valid field.

* src/util/virnodesuspend.c: Ensure *bitmask is zero upon
  failure
* src/conf/capabilities.c, src/conf/capabilities.h: Remove
  powerMgmt_valid field
* src/qemu/qemu_capabilities.c: Remove powerMgmt_valid