core: Remove HasFreeVmsInPool

HasFreeVmsInPoolQuery is not used anywhere in
the code, and hence should be removed.

This patch removes the following:
1. The HasFreeVmsInPoolQuery class
2. The HasFreeVmsInPool constant representing
   it in VdcQueryType
3. The HasFreeVmsInPoolParameters class which
   was only used by the removed query class.
4. Moved several static methods from VmPoolCommandBase which were used
   by HasFreeVmsInPoolQuery and AttachUserToVmFromPoolAndRunCommand to
   AttachUserToVmFromPoolAndRunCommand, as it is now the only command
   that uses them - getVmToAttach(NGuid),
   getPrestartedVmToAttach(NGuid), getNonPrestartedVmToAttach(NGuid).
   These methods' visibility was also reduced to private, as they no
   longer need to be used outside this class.

Change-Id: I3ed0bd42dc7c3c33bffcd97035c5c5e80bdc6df6
Signed-off-by: Allon Mureinik <amureini@redhat.com>