virtio_blk: eliminate anonymous module_init & module_exit

Eliminate anonymous module_init() and module_exit(), which can lead to
confusion or ambiguity when reading System.map, crashes/oops/bugs,
or an initcall_debug log.

Give each of these init and exit functions unique driver-specific
names to eliminate the anonymous names.

Example 1: (System.map)
 ffffffff832fc78c t init
 ffffffff832fc79e t init
 ffffffff832fc8f8 t init

Example 2: (initcall_debug log)
 calling  init+0x0/0x12 @ 1
 initcall init+0x0/0x12 returned 0 after 15 usecs
 calling  init+0x0/0x60 @ 1
 initcall init+0x0/0x60 returned 0 after 2 usecs
 calling  init+0x0/0x9a @ 1
 initcall init+0x0/0x9a returned 0 after 74 usecs

Fixes: e467cde2

 ("Block driver using virtio.")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Cc: Jens Axboe...