qemu_hotplug: Report VIR_ERR_DEVICE_MISSING when device is not found

qemu_hotplug: Report VIR_ERR_DEVICE_MISSING when device is not found

126db34a81bc9f9f9710408f88cceaa1e34bbbd7 had previously switched various
flows over to this from VIR_ERR_OPERATION_FAILED.

This change simply does the same for qemuDomainDetachPrepDisk,
qemuDomainDetachPrepInput and qemuDomainDetachPrepVsock to allow
management apps to centralise their error handling on just
VIR_ERR_DEVICE_MISSING for missing devices during a detach.

Signed-off-by: Lee Yarwood <lyarwood@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>