qemu: monitor: Fix a memory leak in qemuMonitorJSONAttachCharDevCommand

qemu: monitor: Fix a memory leak in qemuMonitorJSONAttachCharDevCommand

With the current logic, we only free @tlsalias as part of the error
label and would have to free it explicitly earlier in the code. Convert
the error label to cleanup, so that we have only one sink, where we
handle all frees. Since JSON object append operation consumes pointers,
make sure @backend is cleared before we hit the cleanup label.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>