Use error_setg() to return an error rather than an explicit exit().
Previously it was an exit(0) instead of a non-zero exit code, which was
simply a bug.  Also improve the error message.

While we're at it change the type of spapr_vga_init() to bool since that's
how we're using it anyway.
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Reviewed-by: NAlexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>