qemuBuildMemoryBackendProps: Prealloc mem for memfd backend
qemuBuildMemoryBackendProps: Prealloc mem for memfd backend

If a domain was using hugepages through memory-backend-file or
via -mem-path, we would turn prealloc on. But we are not doing
that for memory-backend-memfd. Fix this, because we need QEMU to
fully allocate hugepages.

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Reviewed-by: Daniel Henrique Barboza <627b089ad62deae9aaf06ccfa2c9056df28927e0@gmail.com>

jardasgit/libvirt,libvirt/libvirt,libvirt/libvirt,jardasgit/libvirt,zippy2/libvirt,jfehlig/libvirt,zippy2/libvirt,crobinso/libvirt,jardasgit/libvirt,libvirt/libvirt,nertpinx/libvirt,jardasgit/libvirt,zippy2/libvirt,jfehlig/libvirt,nertpinx/libvirt,zippy2/libvirt,crobinso/libvirt,nertpinx/libvirt,olafhering/libvirt,jfehlig/libvirt,olafhering/libvirt,jfehlig/libvirt,libvirt/libvirt,nertpinx/libvirt,crobinso/libvirt,crobinso/libvirt,olafhering/libvirt,jardasgit/libvirt,olafhering/libvirt,nertpinx/libvirt