gui: change margins in create_basic_grid

gui: change margins in create_basic_grid

Previously the grids were inserted in a succession of boxes each with
its own set of margins and spacing. We define the margins now
exclusively in the grid and account for previous values.

Note that we match the top and bottom margins of the 'Target Harddisk'
panel which does not need to use a grid.

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
Reviewed-by: Christoph Heiss <c.heiss@proxmox.com>
Tested-by: Christoph Heiss <c.heiss@proxmox.com>
Tested-by: Tested-by: Lukas Wagner <l.wagner@proxmox.com>