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.