xen-scsiback: define a pr_fmt macro with xen-pvscsi

Add the {xen-pvscsi: } prefix in pr_fmt and remove DPRINTK, then
replace all DPRINTK with pr_debug.

Also fixed up some comments just as eliminate redundant whitespace
and format the code.

These will make the code easier to read.
Signed-off-by: Tao Chen <boby.chen@huawei.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>