add support for configurable VirtualSystemType value in vsphere OVF support

The default value remains "vmx-07 vmx-08"

According to VMWare, this value can be a list of one or more vmx-## values
where the number corresponds to support for the following products:

10 - ESXi 5.5, Fusion 6.x, Workstation 10.x, Player 6.x
9 - ESXi 5.1, Fusion 5.x, Workstation 9.x, Player 5.x
8 - ESXi 5.0, Fusion 4.x, Workstation 8.x, Player 4.x
7 - ESXi/ESX 4.x, Fusion 3.x, Fusion 2.x, Workstation 7.x, Workstation 6.5.x,
    Player 3.x, Server 2.x
(earlier versions omitted)

Detalis taken from:

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1003746

This relates to BZ 1059456:

https://bugzilla.redhat.com/show_bug.cgi?id=1059456
Merge pull request #346 from redhat-imaging/feature/vmware_virt_hw_version_take2

add support for configurable VirtualSystemType value in vsphere OVF supp...