Split qemuDomainChrInsert into two parts

Split qemuDomainChrInsert into two parts

Do the allocation first, then add the actual device.
The second part should never fail. This is good
for live hotplug where we don't want to remove the device
on OOM after the monitor command succeeded.

The only change in behavior is that on failure, the
vmdef->consoles array is freed, not just the first console.