Implement support for allowing cpu and memory updates to the vm resource

This addresses #111 and #40

Todo

[x] Decide the final scope of this change
- ~Should the other missing vm.set attributes be supported?~
- ~Should the breaking change to support the necessary cpu and memory toggles be made?~
- The breaking changes and full support of the vm.set parameters will be done in a later PR
[x] Add test that ensures that VM is not shutdown when cpu change is within cpu max
[x] Add test that ensure that VM is not shutdown when memory change is within memory limits
[x] Clean up the mocking code needed to validate that online cpu and memory changes work as expected
[x] Create github issue to track supporting the other vm.set parameters in the vm resource update (#145)
make testacc and make testclient pass
- [x] make testclient passes
- [x] make acctest passes