vl: exit if a bad property value is passed to -global

When passing '-global driver=host-powerpc64-cpu,property=compat,value=foo'
on the command line, without this patch, we get the following warning per
device (which means many lines if the guests has many cpus):

qemu-system-ppc64: Warning: can't apply global host-powerpc64-cpu.compat=foo:
    Invalid compatibility mode "foo"

... and QEMU continues execution, ignoring the property.

With this patch, we get a single line:

qemu-system-ppc64: can't apply global host-powerpc64-cpu.compat=foo:
    Invalid compatibility mode "foo"

... and QEMU exits.

The previous behavior is kept for hotplugged devices since we don't want
QEMU to exit when doing device_add.

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
