Commit 5b3492fadb moved qemuAgentCheckError calls into
qemuAgentCommand for various reasons; however, subsequent
commit 0977b8aa0 adding a new command made call again
So let's just remove the duplicitous call from
qemuAgentGetInterfaces.
Signed-off-by: John Ferlan <jferlan@redhat.com> ACKed-by: Michal Privoznik <mprivozn@redhat.com>