hw/display/framebuffer.c: Avoid overflow for framebuffers > 4GB

Coverity points out that calculating src_len by multiplying
src_width by rows could overflow. This can only happen in
the implausible case of a framebuffer larger than 4GB, but
we may as well fix it, placating Coverity. (CID1005515)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
