vga: disable global_vmstate for 3.0+ machine types

Move global_vmstate from vga_common_init() parameter to VGACommonState
field.  Set global_vmstate to true for isa vga devices, so nothing
changes here.  virtio-vga and secondary-vga already set global_vmstate
to false so no change here either.  All other pci vga devices get a new
global-vmstate property, defaulting to false.  A compat property flips
it to true for older machine types.

With this in place you don't get a vmstate section naming conflict any
more when adding multiple pci vga devices to your vm.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-Id: <20180702163345.17892-1-kraxel@redhat.com>