checkpatch: Fix whitespace checks for documentation code blocks

Prevent blank lines in documentation code blocks to be signalled as
incorrect trailing whitespace.

Code blocks in documentation are 4-column aligned, and blank lines in
them should have exactly 4 columns of trailing whitespace to prevent
QEMU's wiki to render them as separate code blocks.

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Message-Id: <147325254382.22644.5531276787733455773.stgit@fimbulvetr.bsc.es>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
