libxl: make domain resume API asynchronous

libxl: make domain resume API asynchronous

Although the current implementation has no asynchromous parts I can
envisage it needing to do bits of create/destroy like functionality
which may need async support in the future.

To do this make the meat into an internal libxl__domain_resume
function in order to satisfy the no-internal-callers rule for the
async function.

Since I needed to touch the logging to s/ctx/CTX/ anyway switch to the
LOG* helper macros.

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