bhyve: fix bhyveConnectAgent()

bhyve: fix bhyveConnectAgent()

The bhyveConnectAgent() function calls qemuAgentOpen() to open an agent
connection. If it fails, e.g. because of insufficient permissions to
open the socket, it returns NULL. Currently, if that happens,
bhyveConnectAgent() just sets agentError to true and exits with 0.
This does not match the contract of bhyveDomainEnsureAgent(), which
should either provide an agent connection or fail.

Fix that by returning -1 when qemuAgentOpen() fails. To make intent
clearer, add a documentation for bhyveConnectAgent().

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>