vbox: fix segfault on virsh dumpxml with the existence of USB filters
    
    A USB filter is stored in a hostdev. The original code doesn't
    allocate hostdev->info that is expected to be allocated with hostdev.
    So use virDomainHostdevDefAlloc() to allocate both as we expect.
    
    Signed-off-by: Ryota Ozaki <ozaki.ryota@gmail.com>