libxl: Re-introduce libxl__domain_resume

libxl: Re-introduce libxl__domain_resume

libxl__domain_resume is a rework libxl__domain_resume_deprecated. It
makes uses of ev_xswatch and ev_qmp, to replace synchronous QMP calls
and libxl__wait_for_device_model_deprecated call.

This patch also introduce libxl__dm_resume which is a sub-operation of
both libxl__domain_resume and libxl__domain_unpause and can be used
instead of libxl__domain_resume_device_model_deprecated.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>