This is a strange bug... I assume it's a packaging issue. I experienced this, and also got slightly different output when I tried a reinstall:

[james@purpleidea ~]$ vagrant plugin list
vagrant-hostmanager (1.5.0)
vagrant-login (1.0.1, system)
vagrant-share (1.1.4, system)
[james@purpleidea ~]$ sudo dnf reinstall vagrant-libvirt
[sudo] password for james:
Last metadata expiration check performed 0:07:10 ago on Mon Jul 27 15:23:42 2015.
Dependencies resolved.
================================================================================
 Package                Arch          Version               Repository     Size
================================================================================
Reinstalling:
 vagrant-libvirt        noarch        0.0.26-2.fc22         fedora         57 k

Transaction Summary
================================================================================

Total download size: 57 k
Is this ok [y/N]: y
Downloading Packages:
vagrant-libvirt-0.0.26-2.fc22.noarch.rpm        117 kB/s |  57 kB     00:00
--------------------------------------------------------------------------------
Total                                            11 kB/s |  57 kB     00:05
Running transaction check
Transaction check succeeded.
Running transaction test
Transaction test succeeded.
Running transaction
  Reinstalling: vagrant-libvirt-0.0.26-2.fc22.noarch                        1/2 
Ignoring json-1.8.2 because its extensions are not built.  Try: gem pristine json --version 1.8.2
Ignoring nokogiri-1.6.6.2 because its extensions are not built.  Try: gem pristine nokogiri --version 1.6.6.2
Ignoring ruby-libvirt-0.4.0 because its extensions are not built.  Try: gem pristine ruby-libvirt --version 0.4.0
LoadError: no such file to load -- vagrant/plugin/manager
  require at org/jruby/RubyKernel.java:1071
  require at /usr/share/rubygems/rubygems/core_ext/kernel_require.rb:54
   (root) at -e:3
error: %preun(vagrant-libvirt-0.0.26-2.fc22.noarch) scriptlet failed, exit status 1
Error in PREUN scriptlet in rpm package vagrant-libvirt
  Erasing     : vagrant-libvirt-0.0.26-2.fc22.noarch                        2/2 
Ignoring json-1.8.2 because its extensions are not built.  Try: gem pristine json --version 1.8.2
Ignoring nokogiri-1.6.6.2 because its extensions are not built.  Try: gem pristine nokogiri --version 1.6.6.2
Ignoring ruby-libvirt-0.4.0 because its extensions are not built.  Try: gem pristine ruby-libvirt --version 0.4.0
LoadError: no such file to load -- vagrant/plugin/manager
  require at org/jruby/RubyKernel.java:1071
  require at /usr/share/rubygems/rubygems/core_ext/kernel_require.rb:54
   (root) at -e:3
warning: %posttrans(vagrant-libvirt-0.0.26-2.fc22.noarch) scriptlet failed, exit status 1
Non-fatal POSTTRANS scriptlet failure in rpm package vagrant-libvirt
  Verifying   : vagrant-libvirt-0.0.26-2.fc22.noarch                        1/2 
  Verifying   : vagrant-libvirt-0.0.26-2.fc22.noarch                        2/2 

Reinstalled:
  vagrant-libvirt.noarch 0.0.26-2.fc22                                          

Complete!
[james@purpleidea ~]$ vagrant plugin list
vagrant-hostmanager (1.5.0)
vagrant-login (1.0.1, system)
vagrant-share (1.1.4, system)


Plugin is still not seen :(

(In reply to James (purpleidea) from comment #1)
> LoadError: no such file to load -- vagrant/plugin/manager
>   require at org/jruby/RubyKernel.java:1071
>   require at /usr/share/rubygems/rubygems/core_ext/kernel_require.rb:54
>    (root) at -e:3

JRuby? How that comes?

(In reply to Vít Ondruch from comment #2)
> (In reply to James (purpleidea) from comment #1)
> > LoadError: no such file to load -- vagrant/plugin/manager
> >   require at org/jruby/RubyKernel.java:1071
> >   require at /usr/share/rubygems/rubygems/core_ext/kernel_require.rb:54
> >    (root) at -e:3
> 
> JRuby? How that comes?

No idea. Maybe that system is messed up somehow... But understanding what's happening in the Vanilla scenario that Riek showed would be useful to debug...

I would love to see some concrete steps made on a new system. How did you Daniel got into your state-of-affairs?

Daniel,

can you uninstall vagrant-libvirt with:
# rpm -e --nodeps --noscripts vagrant-libvirt

and then install it again with:
# dnf install vagrant-libvirt

and confirm this is really still an issue?

Fedora 22 changed to end-of-life (EOL) status on 2016-07-19. Fedora 22 is
no longer maintained, which means that it will not receive any further
security or bug fix updates. As a result we are closing this bug.

If you can reproduce this bug against a currently maintained version of
Fedora please feel free to reopen this bug against that version. If you
are unable to reopen this bug, please file a new report against the
current release. If you experience problems, please add a comment to this
bug.

Thank you for reporting this bug and we are sorry it could not be fixed.