conf: Add 'x' and 'y' resolution into video XML definition

conf: Add 'x' and 'y' resolution into video XML definition

This commit adds resolution element with parameters 'x' and 'y' into video
XML domain group definition. Both, properties were added into an element
called 'resolution' and it was added inside 'model' element. They are set
as optional. This element does not follow QEMU properties 'xres' and
'yres' format. Both HTML documentation and schema were changed too. This
commit includes a simple test case to cover resolution for QEMU video
models. The new XML format for resolution looks like:

    <model ...>
      <resolution x='800' y='600'/>
    </model>

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Julio Faracco <jcfaracco@gmail.com>