qemu_hotplug: Allow asynchronous detach

qemu_hotplug: Allow asynchronous detach

The virDomainDetachDeviceAlias API is designed so that it only
sends detach request to qemu. It's user's responsibility to wait
for DEVICE_DELETED event, not libvirt's. Add @async flag to
qemuDomainDetach*Device() functions so that caller can chose if
detach is semi-synchronous (old virDomainDetachDeviceFlags()) or
fully asynchronous (new virDomainDetachDeviceFlags()).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>