qemu: Create a common qemuGetSecretString
qemu: Create a common qemuGetSecretString

Make the secret fetching code common for qemuBuildRBDString() and
qemuBuildDriveURIString() using the virDomainDiskDef.

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