cirrus: add blit_is_unsafe call to cirrus_bitblt_cputovideo

cirrus: add blit_is_unsafe call to cirrus_bitblt_cputovideo

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.

The missing blit width check allows to overflow cirrus_bltbuf,
with the attractive target cirrus_srcptr (current cirrus_bltbuf write
position) being located right after cirrus_bltbuf in CirrusVGAState.

Due to cirrus emulation writing cirrus_bltbuf bytewise the attacker
hasn't full control over cirrus_srcptr though, only one byte can be
changed.  Once the first byte has been modified further writes land
elsewhere.

[ This is CVE-2017-2620 / XSA-209  - Ian Jackson ]

Reported-by: Gerd Hoffmann <ghoffman@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>