libxl: synchronize device removal when using driver domains

libxl: synchronize device removal when using driver domains

Synchronize the clean up of the backend from the toolstack domain when
the driver domain has actually finished closing the backend for the
device.

This is accomplished by waiting for the driver domain to  remove the
directory containing the backend keys, then the toolstack domain will
finish the cleanup by removing the empty folders on the backend path.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>