CODING_STYLE, checkpatch: update line length rules

Line lengths above 80 characters do exist.  They are rare, but
they happen from time to time.  An ignored rule is worse than an
exception to the rule, so do the latter.

Some on the list expressed their preference for a soft limit that
is slightly lower than 80 characters, to account for extra characters
in unified diffs (including three-way diffs) and for email quoting.
However, there was no consensus on this so keep the 80-character
soft limit and add a hard limit at 90.

Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
