bugfix: Change the plugin source name to its valid form.

[khung]

packer init fails with the new plugin source name "github.com/vatesfr/packer-plugin-xenserver", at least on Packer v1.13.1. This is the error message that gets returned:

Error: Failed to parse source

  on ubuntu/xo-ubuntu.pkr.hcl line 3, in packer:
   3:     xenserver = {
   4:       version = "~> v0.8.1"
   5:       source  = "github.com/vatesfr/packer-plugin-xenserver"
   6:     }

Plugin source has a type with the prefix "packer-plugin-", which isn't valid.
Although that prefix is often used in the names of version control repositories
for Packer plugins, plugin source strings should not include it.

Did you mean "xenserver"?