x86/xstate: fix xcomp_bv initialization

x86/xstate: fix xcomp_bv initialization

We must not clear the compaction bit when using XSAVES/XRSTORS. And
we need to guarantee that xcomp_bv never has any bits clear which
are set in xstate_bv (which requires partly undoing commit 83ae0bb226
["x86/xsave: simplify xcomp_bv initialization"]). Split initialization
of xcomp_bv from the other FPU/SSE/AVX related state setup in
arch_set_info_guest() and hvm_load_cpu_ctxt().

Reported-by: Harmandeep Kaur <write.harmandeep@gmail.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>