qemu: job usage issue in several APIs leading to libvirtd crash

Multiple race conditions in the 1 virDomainBlockStats, 2 virDomainGetBlockInf, 3 qemuDomainBlockJobImpl, and 4 virDomainGetBlockIoTune functions in libvirt before 1.2.1 do not properly verify that the disk is attached, which allows remote read-only attackers to cause a denial of service libvirtd...