x86/xsave: recover from faults on XRSTOR

x86/xsave: recover from faults on XRSTOR

Just like FXRSTOR, XRSTOR can raise #GP if bad content is being passed
to it in the memory block (i.e. aspects not under the control of the
hypervisor, other than e.g. proper alignment of the block).

Also correct the comment explaining why FXRSTOR needs exception
recovery code to not wrongly state that this can only be a result of
the control tools passing a bad image.

This is CVE-2013-2077 / XSA-53.

Signed-off-by: Jan Beulich <jbeulich@suse.com>