vl: Use error_report() for --display errors

Results in nicer error messages.  Before this patch:

    Invalid GTK option string: gtk,lirum-larum

After:

    qemu-system-x86_64: -display gtk,lirum-larum: Invalid GTK option string

Of course, the thing ought to use QemuOpts instead of parsing by hand.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
