Add integ tests for Vm#update_cpu_count

Decrease and increase CPU count. Like Vm#memory, this highlights that Vm#cpu
returns a string :(
Add Vm#update_metadata integ tests

Test that metadata updates preserve existing metadata.

Add a test to highlight that Vm#update_metadata also updates parent vApp metadata.
Add Vm#add_extra_disks integration tests

This also adds a helper function to get *just* the disks returned by
(Model) vm.disks -- it returns the disk controllers (SCSI+ID) too, which
we can filter out by matching on name =~ /^Hard Disk/