vga: do not dynamically allocate chain4_alias

vga: do not dynamically allocate chain4_alias

Instead, add a boolean variable to indicate the presence of the region.
This avoids a repeated malloc/free (later we can also avoid the
add_child/unparent by changing the offset/size of the alias).

Reviewed-by: Peter Crosthwaite <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>