x86emul: correct handling of FPU insns faulting on memory write

x86emul: correct handling of FPU insns faulting on memory write

When an FPU instruction with a memory destination fails during the
memory write, it should not affect FPU register state. Due to the way
we emulate FPU (and SIMD) instructions, we can only guarantee this by
- backing out changes to the FPU register state in such a case or
- doing a descriptor read and/or page walk up front, perhaps with the
  stubs accessing the actual memory location then.
The latter would require a significant change in how the emulator does
its guest memory accessing, so for now the former variant is being
chosen.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com> [hvm/emulate.c]
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>