libxl: Re-implement domain_suspend_device_model using libxl__ev_qmp

libxl: Re-implement domain_suspend_device_model using libxl__ev_qmp

The re-implementation is done because we want to be able to send the
file description that QEMU can use to save its state. When QEMU is
restricted, it would not be able to write to a path.

This replace both libxl__qmp_stop() and libxl__qmp_save().

qmp_qemu_check_version() was only used by libxl__qmp_save(), so it is
replace by a version using libxl__ev_qmp instead.

Coding style fixed in libxl__domain_suspend_device_model() for the
return value.

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