cirrus: fix blit region check

Issues:
 * Doesn't check pitches correctly in case it is negative.
 * Doesn't check width at all.

Turn macro into functions while being at it, also factor out the check
for one region which we then can simply call twice for src + dst.

This is CVE-2014-8106.

Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>