Reboot the VM after installing new additions.

[gregsymons]

This prevents problems with doing things that require the guest additions like
setting up shared folders or port forwardings.

[gregsymons]

This fixes #24

[fnordfish]

Hi @gregsymons,

thanks for your input! I was very unsure if users would like the box to get rebooted.
I've merged your PR, even if github is telling otherwise, but had to ad some more guards and options.

Thanks again for contributing.

[gregsymons]

I looked at your changes and the only concern I have is calling @vm.reload instead of calling Vagrant::Action::VM::Halt and Vagrant::Action::VM::Boot. I originally did @vm.reload, and found that doing so led to provisioners (in my case the Chef::Solo provisioner) running twice--once during reload, and once after the plugin yields to the next action.

[fnordfish]

@gregsymons you are right, provisioning will be executed twice - which should not happen, of cause.