qemuDomainBlockStatsFlags: Guard disk lookup with a domain job
qemuDomainBlockStatsFlags: Guard disk lookup with a domain job

When there are two concurrent threads, we may dereference a NULL
pointer, even though it has been checked before:

1. Thread1: starts executing qemuDomainBlockStatsFlags() with nparams != 0.
            It finds given disk and successfully pass check for disk->info.alias
            not being NULL.
2. Thread2: starts executing qemuDomainDetachDeviceFlags() on the very same
            disk as Thread1 is working on.
3. Thread1: gets to qemuDomainObjBeginJob() where it sets a job on a
            domain.
4. Thread2: also tries to set a job. However, we are not guaranteed which
            thread wins. So assume it's Thread2 who can continue.
5. Thread2: does the actual detach and frees disk->info.alias
6. Thread2: quits the job
7. Thread1: now successfully acquires the job, and accesses a NULL pointer.

olafhering/libvirt,jfehlig/libvirt,andreabolognani/libvirt,crobinso/libvirt,cbosdo/libvirt,datto/libvirt,cbosdo/libvirt,andreabolognani/libvirt,nertpinx/libvirt,elmarco/libvirt,agx/libvirt,jardasgit/libvirt,datto/libvirt,eskultety/libvirt,cbosdo/libvirt,VenkatDatta/libvirt,eskultety/libvirt,datto/libvirt,datto/libvirt,VenkatDatta/libvirt,rlaager/libvirt,fabianfreyer/libvirt,rlaager/libvirt,eskultety/libvirt,shugaoye/libvirt,eskultety/libvirt,zippy2/libvirt,taget/libvirt,nertpinx/libvirt,libvirt/libvirt,andreabolognani/libvirt,crobinso/libvirt,fabianfreyer/libvirt,crobinso/libvirt,taget/libvirt,agx/libvirt,VenkatDatta/libvirt,shugaoye/libvirt,zippy2/libvirt,VenkatDatta/libvirt,taget/libvirt,olafhering/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,libvirt/libvirt,olafhering/libvirt,agx/libvirt,zippy2/libvirt,elmarco/libvirt,shugaoye/libvirt,olafhering/libvirt,elmarco/libvirt,jardasgit/libvirt,shugaoye/libvirt,libvirt/libvirt,jardasgit/libvirt,andreabolognani/libvirt,elmarco/libvirt,zippy2/libvirt,agx/libvirt,jardasgit/libvirt,rlaager/libvirt,zhlcindy/libvirt-1.1.4-maintain,datto/libvirt,zhlcindy/libvirt-1.1.4-maintain,jfehlig/libvirt,nertpinx/libvirt,zhlcindy/libvirt-1.1.4-maintain,jfehlig/libvirt,nertpinx/libvirt,nertpinx/libvirt,andreabolognani/libvirt,elmarco/libvirt,libvirt/libvirt,fabianfreyer/libvirt,cbosdo/libvirt,taget/libvirt,agx/libvirt,fabianfreyer/libvirt,eskultety/libvirt,cbosdo/libvirt,zhlcindy/libvirt-1.1.4-maintain,rlaager/libvirt,zhlcindy/libvirt-1.1.4-maintain,crobinso/libvirt,VenkatDatta/libvirt,rlaager/libvirt,shugaoye/libvirt,jardasgit/libvirt,taget/libvirt