Move the VNC cap validation from qemuBuildGraphicsVNCCommandLine()
to qemuDomainDeviceDefValidateGraphics(). This function is called by
qemuDomainDefValidate(), validating the graphics parameters in domain
define time.

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>