pseries: Clean up error handling in spapr_vga_init()

pseries: Clean up error handling in spapr_vga_init()

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: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Markus Armbruster <armbru@redhat.com>