fix #7525: vga: allow setting the number of outputs for VirtIO GPU

With SPICE and QXL, Linux guests get 'max_outputs=4' on the display
device, so they can use up to four displays with a single device. For
the VirtIO GPU, there is no way to change QEMU's default of a single
output, so using multiple displays requires passing the whole device
via custom arguments.

Add a 'max-outputs' option to the 'vga' property, which sets the
'max_outputs' property of the VirtIO GPU, for both 'virtio' and
'virtio-gl'. QEMU supports up to 16 outputs. Other display types
reject the option, as QXL handles multiple displays on its own.

Signed-off-by: Michal Fox <me@dualfroz.com>

src/PVE/QemuServer.pm

@@ -187,6 +187,15 @@ my $vga_fmt = {
         enum => ['vnc'],
         optional => 1,
     },
+    'max-outputs' => {
+        description => "Sets the maximum number of displays the guest can use. Only supported"
+            . " with 'virtio' and 'virtio-gl'.",
+        type => 'integer',
+        optional => 1,
+        minimum => 1,
+        maximum => 16,
+        default => 1,
+    },
 };
 
 my $ivshmem_fmt = {
@@ -1513,6 +1522,12 @@ sub print_vga_device {
         }
     }
 
+    if (my $outputs = $vga->{'max-outputs'}) {
+        die "'max-outputs' is only supported with VGA type 'virtio' or 'virtio-gl'\n"
+            if $vga->{type} !~ m/^virtio/;
+        $max_outputs = ",max_outputs=$outputs";
+    }
+
     die "no device-type for $vga->{type}\n" if !$type;
 
     my $memory = "";