tools/save: Drop unused parameters from xc_domain_save()

tools/save: Drop unused parameters from xc_domain_save()

XCFLAGS_CHECKPOINT_COMPRESS has been unused since c/s b15bc4345 (2015),
XCFLAGS_HVM since c/s 9e8672f1c (2013), and XCFLAGS_STDVGA since c/s
087d43326 (2007).  Drop the constants, and code which sets them.

The separate hvm parameter (appeared in c/s d11bec8a1, 2007 and ultimately
redundant with XCFLAGS_HVM), is used for sanity checking and debug printing,
then discarded and replaced with Xen's idea of whether the domain is PV or
HVM.

Rearrange the logic in xc_domain_save() to ask Xen sightly earlier, and use a
consistent idea of 'hvm' throughout.  Removing this parameter removes the
final user of libxl's dss->hvm, so drop that field as well.

Update the doxygen comment to be accurate.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <Ian.Jackson@citrix.com>