hw/display/vga: Report a proper error when adding a 2nd ISA VGA

hw/display/vga: Report a proper error when adding a 2nd ISA VGA

QEMU currently abort()s if the user tries to add a second ISA VGA
device, for example:

$ ./qemu-system-x86_64 -device isa-vga -device isa-vga
RAMBlock "vga.vram" already registered, abort!
Aborted (core dumped)
$ ./qemu-system-x86_64 -device isa-cirrus-vga -device isa-cirrus-vga
RAMBlock "vga.vram" already registered, abort!
Aborted (core dumped)
$ ./qemu-system-mips64el -M pica61 -device isa-vga
RAMBlock "vga.vram" already registered, abort!
Aborted (core dumped)

Such a crash should never happen just because of giving bad parameters
at the command line. Let's return a proper error message instead.
(The idea is based on an original patch by Jose R. Ziviani for the
isa-vga device, but this now fixes it for the isa-cirrus-vga device, too)

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/44
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220317083027.16688-4-thuth@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>