qxl: change the default value for vgamem_mb to 16 MiB

qxl: change the default value for vgamem_mb to 16 MiB

The default value should be 16 MiB instead of 8 MiB. Only really old
version of upstream QEMU used the 8 MiB as default for vga framebuffer.

Without this change if you update your libvirt where we introduced the
"vgamem" attribute for QXL video device the value will be set to 8 MiB,
but previously your guest had 16 MiB because we didn't pass any value to
QEMU command line which means QEMU used its own 16 MiB as default.

This will affect all users with guest's display resolution higher than
1920x1080.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>