vbox_storage: fix coverity issue with overwriting value
    
    Coverity is complaining about overwriting value in 'rc' variable
    without using the old value because it somehow doesn't recognize that
    the value is used by MACRO. The 'rc' variable is there only for checking
    return code so it's save to remove it and make coverity happy.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>