x86: consolidate legacy FPU state loading

x86: consolidate legacy FPU state loading

First of all introduce a helper function instead of replicating almost
the same code for PV and HVM. The differences between the two pieces of
code actually points out an issue (which is also addressed here): In
the HVM case FCW would not have been set to FCW_RESET in certain cases
(note for example that XRSTOR loads FCW_DEFAULT rather then FCW_RESET
when the respective xstate_bv bit is clear).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>