Fix potential deadlock when agent is closed

Fix potential deadlock when agent is closed

If the qemuAgentClose method is called from a place which holds
the domain lock, it is theoretically possible to get a deadlock
in the agent destroy callback. This has not been observed, but
the equivalent code in the QEMU monitor destroy callback has seen
a deadlock.

Remove the redundant locking while unrefing the object and the
bogus assignment

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>