Add method to fetch virtual devices attached to VM on VMware

[Gauravtalreja1]

Results:

In [1]: from wrapanapi import VMWareSystem
   ...: from wrapanapi.systems.virtualcenter import VMWareVirtualMachine

In [2]: vmwareclient = VMWareSystem(
   ...:         hostname=vmware.hostname,
   ...:         username=vmware.username,
   ...:         password=vmware.password,
   ...:     )
In [3]: vm = vmwareclient.get_vm('heidi-detten.example.com')
In [4]: vm.get_virtual_device_type_names()
Out[4]:
['VirtualController',
 'VirtualKeyboard',
 'VirtualPointingDevice',
 'VirtualVideoCard',
 'VirtualVMCIDevice',
 'VirtualDisk',
 'VirtualEthernetCard',
 'VirtualTPM']