qga: add *reset argument to ssh-add-authorized-keys

I prefer 'reset' over 'clear', since 'clear' and keys may have some
other relations or meaning.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
*fix disallowed g_assert* usage reported by checkpatch
Signed-off-by: Michael Roth <michael.roth@amd.com>