qemuBuildInterfaceCommandLine: Use qemuFDPass for the vdpa fd
qemuBuildInterfaceCommandLine: Use qemuFDPass for the vdpa fd

Use the new helpers for passing of the file descriptor needed for 'vdpa'
interfaces.

Apart from the simplification in this case it will allow further changes
to unify all fdset handling.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

zippy2/libvirt,zippy2/libvirt,olafhering/libvirt,jfehlig/libvirt,jfehlig/libvirt,olafhering/libvirt,libvirt/libvirt,crobinso/libvirt,olafhering/libvirt,crobinso/libvirt,libvirt/libvirt,libvirt/libvirt,crobinso/libvirt,crobinso/libvirt,libvirt/libvirt,olafhering/libvirt,zippy2/libvirt,jfehlig/libvirt,zippy2/libvirt,jfehlig/libvirt