x86/domctl: Fix migration of guests which are not using xsave

x86/domctl: Fix migration of guests which are not using xsave

c/s da62246e "x86/xsaves: enable xsaves/xrstors/xsavec in xen" broke migration
of PV guests which were not using xsave.

In such a case, compress_xsave_states() gets passed a zero length buffer.  The
first thing it tries to do is ASSERT() on user-provided data, if it hadn't
already wandered off the end of the buffer to do so.

Perform more verification of the input buffer before passing it to
compress_xsave_states().  This involves making xsave_area_compressed() public.

Similar problems exist on the HVM side, so make equivalent adjustments there.
This doesn't manifest in general, as hvm_save_cpu_xsave_states() elides the
entire record if xsave isn't used, but is a problem if a caller were to
construct an xsave record manually.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <JBeulich@suse.com>