cirrus: add blit_is_unsafe call to cirrus_bitblt_cputovideo (CVE-2017-2620)

CIRRUS_BLTMODE_MEMSYSSRC blits do NOT check blit destination
and blit width, at all.  Oops.  Fix it.

Security impact: high.

The missing blit destination check allows to write to host memory.
Basically same as CVE-2014-8106 for the other blit variants.

Cc: qemu-stable@nongnu.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
