Avoid shadowed declaration

[jserv]

Reported by Cppcheck:

Checking src/virtio-blk.c ...
src/virtio-blk.c:115:21: style: Local variable 'r' shadows outer variable [shadowVariable]
            ssize_t r;
                    ^
src/virtio-blk.c:105:13: note: Shadowed declaration
        int r = 0;
            ^
src/virtio-blk.c:115:21: note: Shadow variable
            ssize_t r;
                    ^

Command:

$ cppcheck --enable=all src/*.[ch] src/arch/x86/*.[ch]