qemu_hotplug: Fix a rare race condition when detaching a device twice

qemu_hotplug: Fix a rare race condition when detaching a device twice

https://bugzilla.redhat.com/show_bug.cgi?id=1623389

If a device is detached twice from the same domain the following
race condition may happen:

1) The first DetachDevice() call will issue "device_del" on qemu
monitor, but since the DEVICE_DELETED event did not arrive in
time, the API ends claiming "Device detach request sent
successfully".

2) The second DetachDevice() therefore still find the device in
the domain and thus proceeds to detaching it again. It calls
EnterMonitor() and qemuMonitorSend() trying to issue "device_del"
command again. This gets both domain lock and monitor lock
released.

3) At this point, qemu sends us the DEVICE_DELETED event which is
going to be handled by the event loop which ends up calling
qemuDomainSignalDeviceRemoval() to determine who is going to
remove the device from domain definition. Whether it is the
caller that marked the device for removal or whether it is going
to be the event processing thread.

4) Because the device was marked for removal,
qemuDomainSignalDeviceRemoval() returns true, which means the
event is to be processed by the thread that has marked the device
for removal (and is currently still trying to issue "device_del"
command)

5) The thread finally issues the "device_del" command, which
fails (obviously) and therefore it calls
qemuDomainResetDeviceRemoval() to reset the device marking and
quits immediately after, NOT removing any device from the domain
definition.

At this point, the device is still present in the domain
definition but doesn't exist in qemu anymore. Worse, there is no
way to remove it from the domain definition.

Solution is to note down that we've seen the event and if the
second "device_del" fails, not take it as a failure but carry on
with the usual execution.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
ACKed-by: Peter Krempa <pkrempa@redhat.com>