ui: Make qemu_default_pixman_format() return 0 on unsupported formats

In order to remove the logic for detecting supported shared
pixmap formats from device models, make qemu_default_pixman_format()
capable for failing by returning 0 which is not a possible format
value rather than asserting.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
