tools/kvm_stat: move functions to corresponding classes

Quite a few of the functions are used only in a single class. Moving
functions accordingly to improve the overall structure.
Furthermore, introduce a base class for the providers, which might also
come handy for future extensions.
Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>