cirrus: switch to 4 MB video memory by default

Quoting cirrus source code:
   Follow real hardware, cirrus card emulated has 4 MB video memory.
   Also accept 8 MB/16 MB for backward compatibility.

So just use 4MB by default.  We decided to leave that at 8MB by default
a while ago, for live migration compatibility reasons.  But we have
compat properties to handle that, so that isn't a compeling reason.

This also removes some sanity check inconsistencies in the cirrus code.
Some places check against the allocated video memory, some places check
against the 4MB physical hardware has.  Guest code can trigger asserts
because of that.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1489494514-15606-1-git-send-email-kraxel@redhat.com
