monitor: allow device_del to accept QOM paths

Currently device_del requires that the client provide the
device short ID. device_add allows devices to be created
without giving an ID, at which point there is no way to
delete them with device_del. The QOM object path, however,
provides an alternative way to identify the devices.

Allowing device_del to accept an object path ensures all
devices are deletable regardless of whether they have an
ID.

 (qemu) device_add usb-mouse
 (qemu) qom-list /machine/peripheral-anon
 device[0] (child<usb-mouse>)
 type (string)
 (qemu) device_del /machine/peripheral-anon/device[0]

Devices are required to be marked as hotpluggable
otherwise an error is raised

 (qemu) device_del /machine/unattached/device[4]
 Device 'PIIX3' does not support hotplugging

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1441974836-17476-1-git-send-email-berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[Commit message touched up, accidental white-space change dropped]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
diff --git a/hmp-commands.hx b/hmp-commands.hx
index a511004..3a4ae39 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -676,7 +676,8 @@
 STEXI
 @item device_del @var{id}
 @findex device_del
-Remove device @var{id}.
+Remove device @var{id}. @var{id} may be a short ID
+or a QOM object path.
 ETEXI
 
     {