hw/qxl: guest bug on primary create with stride %4 != 0

Due to usage of pixman for rendering on all spice surfaces we have
pixman's requirement that the stride be word aligned. A guest not
honoring that can crash spice and qemu with it due to failure to create
a surface (in spice-server). Avoid this early on in primary surface
creation and offscreen surface creation.

Recently windows guests got odd width support which triggers a non word
aligned primary surface in 16bit color depth. Off screen surfaces have
always been word aligned, but doesn't hurt to check them here too.

Signed-off-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
