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