bochs-display: Fix vgamem=SIZE error handling

bochs-display: Fix vgamem=SIZE error handling

bochs_display_realize() rejects out-of-range vgamem.  The error
handling is broken:

    $ qemu-system-x86_64 -S -display none -monitor stdio
    QEMU 4.2.93 monitor - type 'help' for more information
    (qemu) device_add bochs-display,vgamem=1
    Error: bochs-display: video memory too small
    (qemu) device_add bochs-display,vgamem=1
    RAMBlock "0000:00:04.0/bochs-display-vram" already registered, abort!
    Aborted (core dumped)

Cause: bochs_display_realize() neglects to bail out after setting the
error.  Fix that.

Fixes: 765c94290863eef1fc4a67819d452cc13b7854a1
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200422130719.28225-8-armbru@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>