Fix vbox build if name contains spaces

If the vbox name contains spaces, the build process stops with an error in the following cases:
* setting a vm flag
* exporting a box file

Build environment:
Host: Windows 7 (64 bit)

Steps to reproduce:
- create a definition which name contains spaces
- set at least one vm_options in definition.rb
- build the vm

Example:

    bundle exec veewee vbox build "TEST ME"

Output:

    Creating vm TEST ME : 1024M - 1 CPU - Ubuntu_64
    Setting VM Flag usb to on
    Error: We executed a shell command and the exit status was not 0
    - Command :"C:\Program Files\Oracle\VirtualBox\VBoxManage.exe" modifyvm TEST ME --usb on.
    - Exitcode :1.
    - Output   :
    VBoxManage.exe: error: Could not find a registered machine named 'TEST'
    VBoxManage.exe: error: Details: code VBOX_E_OBJECT_NOT_FOUND (0x80bb0001), component VirtualBox, interface IVirtualBox, callee IUnknown
    VBoxManage.exe: error: Context: "FindMachine(Bstr(a->argv[0]).raw(), machine.asOutParam())" at line 455 of file VBoxManageModifyVM.cpp
    Wrong exit code for command "C:\Program Files\Oracle\VirtualBox\VBoxManage.exe"
    modifyvm TEST ME --usb on