Force at least one retry in vm_exists? for provider VirtualBox 4.3

Due to VirtualBox 13190, on Windows, `VBoxManager showvminfo` crashes if the VM is suspended (and maybe in other occasions). This leads Vagrant to think the VM does not exist. This has bad consequences for lot of commands since machine uuid is cleared which leads to a new VM being started. Since there is already a retry mechanism in place, simply force at least one retry of the command. The second invocation should work correctly if the machine is present.

Downside of this patch, it takes 2 more seconds for the UI to respond if the vm really does not exist. We could remove this by forcing a retry without a sleep.