_virtualboxCreateMachine: Avoid unbounded stack
    
    If the stars are in the right position and you're building with
    VBox >= 4.2.0 it will happen that compiler thinks an array
    allocated on the stack may be unbounded:
    
    In file included from vbox/vbox_V4_2.c:13:0:
    vbox/vbox_tmpl.c: In function '_virtualboxCreateMachine':
    vbox/vbox_tmpl.c:2811:1: error: stack usage might be unbounded [-Werror=stack-usage=]
     _virtualboxCreateMachine(vboxGlobalData *data, virDomainDefPtr def, IMachine **machine, char *uuidstr ATTRIBUTE_UNUSED)
     ^
    
    Well, given how the variable is declared, I had some hard time
    seeing it is actually bounded. Surprisingly compiler does not
    complain because of -Wframe-larger-than. This is because
    variable length arrays do not count into that warning.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>