If a device has entries in its holder directory
(/sys/block/$name/holders), it means that some kernel system "uses"
that device, and hence should not be considered available.

This patch adds a new 'in-use' check based on this sysfs test, and
introduces an overal InUse function for devices.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>