nodedev: add helper functions to remove node devices

nodedev: add helper functions to remove node devices

When a mediated device is stopped or undefined by an application outside
of libvirt, we need to remove it from our list of node devices within
libvirt. This patch introduces virNodeDeviceObjListRemoveLocked() and
virNodeDeviceObjListForEachRemove() (which are analogous to other types
of object lists in libvirt) to facilitate that. They will be used in
coming commits.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>