core: cleanup in RunVmCommand

- Extract code sections that initialize command context for child
  commands to separate methods
- Extract code sections that initialize parameters for child commands to
  separate methods
- Change logs to more standard way
- Add static final String member that contains stateless snapshot
  description instead of having explicit string inside the code
- This patch also contains trivial refactoring in VmPoolMonitor

Change-Id: I95c423151b2cdb36d5bd50c635439bea2cec816c
Signed-off-by: Arik Hadas <ahadas@redhat.com>