qemu: change args to qemuAssignDeviceHostdevAlias()
    
    In certain cases, we need to assign a hostdevN-style alias in a case
    when we don't have a virDomainHostdevDefPtr (instead we have a
    virDomainNetDefPtr). Since qemuAssignDeviceHostdevAlias() doesn't use
    anything in the virDomainHostdevDef except the alias string itself
    anyway, this patch just changes the arguments to pass a pointer to the
    alias pointer instead.