Use ps keyword command instead of cmd

For Darwin operating systems, `cmd` is not a valid keyword, thus
resulting in the following error when executing Vagrant `reload` and
`destroy` commands:

    ps: cmd: keyword not found
    ps: no valid keywords; valid keywords:

This commit ensures that SSH sessions for forwarded ports are
terminated via their respective process ID for Darwin *and* Linux
operating systems.

Both Darwin and Linux operating systems support the `command` keyword,
as reflected in the following documents, respectively:

- https://apple.co/2H7jFwl
- https://bit.ly/2HakfW0

lib/vagrant-libvirt/action/forward_ports.rb

@@ -210,9 +210,9 @@ module VagrantPlugins
         end
 
         def ssh_pid?(pid)
-          @logger.debug 'Checking if #{pid} is an ssh process '\
+          @logger.debug "Checking if #{pid} is an ssh process "\
-                        'with `ps -o cmd= #{pid}`'
+                        "with `ps -o command= #{pid}`"
-          `ps -o cmd= #{pid}`.strip.chomp =~ /ssh/
+          `ps -o command= #{pid}`.strip.chomp =~ /ssh/
         end
 
         def remove_ssh_pids