bhyve: add vga configuration for video driver

Add support for vgaconf driver configuration. In domain xml it looks like
this:

  <video>
    <driver vgaconf='io|on|off'>
    <model .../>
  </video>

It was added with bhyve gop video in mind to allow users control how the
video device is exposed to the guest, specifically, how VGA I/O is
handled.

One can refer to the bhyve manual page to get more detailed description
of the possible VGA configuration options:

https://www.freebsd.org/cgi/man.cgi?query=bhyve&manpath=FreeBSD+12-current

The relevant part could be found using the 'vgaconf' keyword.

Also, add some tests for this new feature.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>

tests/bhyvexml2argvtest.c

@@ -193,6 +193,9 @@ mymain(void)
     DO_TEST("net-e1000");
     DO_TEST("uefi");
     DO_TEST("vnc");
+    DO_TEST("vnc-vgaconf-on");
+    DO_TEST("vnc-vgaconf-off");
+    DO_TEST("vnc-vgaconf-io");
 
     /* Address allocation tests */
     DO_TEST("addr-single-sata-disk");