Warn user if the vga flag is passed but no vga device is created

A global boolean variable "vga_interface_created"(declared in softmmu/globals.c)
has been used to track the creation of vga interface. If the vga flag is passed
in the command line "default_vga"(declared in softmmu/vl.c) variable is set to 0.
To warn user, the condition checks if vga_interface_created is false
and default_vga is equal to 0. If "-vga none" is passed, this patch will not warn the
user regarding the creation of VGA device.

The warning "A -vga option was passed but this
machine type does not use that option; no VGA device has been created"
is logged if vga flag is passed but no vga device is created.

This patch has been tested for x86_64, i386, sparc, sparc64 and arm boards.

Signed-off-by: Gautam Agrawal <gautamnagrawal@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/581


Message-Id: <20220501122505.29202-1-gautamnagrawal@gmail.com>
[thuth: Fix wrong warning with "-device" in some cases as reported by Paolo]
Signed-off-by: Thomas Huth <thuth@redhat.com>