libxl: add a transaction parameter to libxl__device_generic_add

libxl: add a transaction parameter to libxl__device_generic_add

Add a xs_transaction_t parameter to libxl__device_generic_add, if it is
XBT_NULL, allocate a proper one.

Update all the callers.

This patch contains a large number of unchecked xenstore operations, we
might want to fix this in the future.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>