denver init fails when Virtualbox is not properly installed

Hi,

Denver crashes on 'denver init' (when Virtualbox kernel module are not loaded)

```
$ ./denver init
panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
denver/pkg/providers.(*Virtualbox).isVMPresent(0xc00017e0b0, 0xc0001b2000, 0x14a, 0x3)
/home/olivier/dev/d3nver/src/pkg/providers/virtualbox.go:371 +0x25d
denver/pkg/providers.(*Virtualbox).checkIfRunning(0xc00017e0b0, 0xc00016473b, 0xefde20, 0xc000133af8)
/home/olivier/dev/d3nver/src/pkg/providers/virtualbox.go:349 +0xcc
denver/pkg/providers.(*Probe).probe(0xc000162dc0, 0xb31dc0, 0xc00017e0b0, 0x0, 0x0)
/home/olivier/dev/d3nver/src/pkg/providers/probe.go:60 +0xaa
denver/pkg/providers.(*Probe).Start(0xc000162dc0, 0xb31dc0, 0xc00017e0b0, 0xa, 0xc000133bc8)
/home/olivier/dev/d3nver/src/pkg/providers/probe.go:30 +0x68
denver/cmd/root.(*Denver).initProbe(...)
/home/olivier/dev/d3nver/src/cmd/root/root.go:190
denver/cmd/root.(*Denver).addBootstrapFunc.func4(0x0, 0x0)
/home/olivier/dev/d3nver/src/cmd/root/root.go:211 +0xa9
denver/cmd/root.(*Denver).Execute.func1(0xc000166280, 0xf16648, 0x0, 0x0, 0x0, 0x0)
/home/olivier/dev/d3nver/src/cmd/root/root.go:86 +0x6e
github.com/spf13/cobra.(*Command).execute(0xc000166280, 0xf16648, 0x0, 0x0, 0xc000166280, 0xf16648)
/home/olivier/go/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:805 +0x56b
github.com/spf13/cobra.(*Command).ExecuteC(0xc000166000, 0xc000146130, 0x1, 0x1)
/home/olivier/go/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:914 +0x2fb
github.com/spf13/cobra.(*Command).Execute(...)
/home/olivier/go/pkg/mod/github.com/spf13/cobra@v0.0.5/command.go:864
denver/cmd/root.(*Denver).Execute(0xc00015e000, 0xa74790)
/home/olivier/dev/d3nver/src/cmd/root/root.go:93 +0x37c
main.main()
```
This may be related to VBoxManage output parsing :

```
VBoxManage list runningvms
WARNING: The vboxdrv kernel module is not loaded. Either there is no module
available for the current kernel (5.3.1-arch1-1-ARCH) or it failed to
load. Please recompile the kernel module and install it by

sudo /sbin/vboxconfig

You will not be able to start VMs until this problem is fixed.
```

Solved by loading Virtualbox modules 'sudo modprobe vboxdrv" and running again "denver init"