qemu: domain: Use qemuDomainSecretInfoNewPlain only for unencrypted secrets
qemu: domain: Use qemuDomainSecretInfoNewPlain only for unencrypted secrets

Move the logic that determines which secret shall be used into the
caller and make this function work only for plain secrets.

This untangles the control flow by only checking relevant data.

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

libvirt/libvirt,jfehlig/libvirt,nertpinx/libvirt,zippy2/libvirt,crobinso/libvirt,jardasgit/libvirt,nertpinx/libvirt,andreabolognani/libvirt,eskultety/libvirt,eskultety/libvirt,jfehlig/libvirt,eskultety/libvirt,zippy2/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,nertpinx/libvirt,libvirt/libvirt,nertpinx/libvirt,jfehlig/libvirt,eskultety/libvirt,nertpinx/libvirt,olafhering/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,jardasgit/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,olafhering/libvirt,libvirt/libvirt,eskultety/libvirt,olafhering/libvirt,libvirt/libvirt,zippy2/libvirt,crobinso/libvirt,crobinso/libvirt,crobinso/libvirt,olafhering/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,zippy2/libvirt