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>
cirrus: don't overflow CirrusVGAState->cirrus_bltbuf

This is CVE-2014-8106.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Merge remote-tracking branch 'remotes/kraxel/tags/pull-cve-2014-8106-20141204-1' into staging

cirrus: fix blit region check

# gpg: Signature made Thu 04 Dec 2014 11:54:57 GMT using RSA key ID D3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>"
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>"
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>"

* remotes/kraxel/tags/pull-cve-2014-8106-20141204-1:
  cirrus: don't overflow CirrusVGAState->cirrus_bltbuf
  cirrus: fix blit region check

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>