xen-blkback: define pr_fmt macro to avoid the duplication of DRV_PFX
Define pr_fmt macro with {xen-blkback: } prefix, then remove all use
of DRV_PFX in the pr sentences. Replace all DPRINTK with pr sentences,
and get rid of DPRINTK macro. It will simplify the code.
And if the pr sentences miss a \n, add it in the end. If the DPRINTK
sentences have redundant \n, remove it. It will format the code.
These all make the readability of the code become better.