qemu: require reply from guest agent in qemuAgentGetInterfaces

qemu: require reply from guest agent in qemuAgentGetInterfaces

Since its introduction in commit 0977b8aa071 (released in v1.2.14)
qemuAgentGetInterfaces calls qemuAgentCommand with needReply=false,
which allows qemuAgentCommand to return 0 even when it did not get
any reply from the agent.

Set needReply to true, since we dereference it right after.

This can be hit if libvirt is waiting for an event from the agent
(e.g. shutdown) and the agent cannot reply in time (e.g. due to
the guest being shut down), as reported in:
https://bugzilla.redhat.com/show_bug.cgi?id=1663051

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>