There are validations for SDL, VNC, SPICE and EGL_HEADLESS
around several BuildGraphics*CommandLine in qemu_command.c. This
patch starts to move all of them to qemu_domain.c, inside the
existent qemuDomainDeviceDefValidateGraphics() function. This
function is called by qemuDomainDefValidate(), validating the
graphics parameters in domain define time.

In this patch we'll move the SDL validation code from
qemuBuildGraphicsSDLCommandLine(). Tests were adapted to consider
SDL validation in this earlier stage.
Reviewed-by: NCole Robinson <crobinso@redhat.com>
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>