x86emul: abstract out XCRn accesses

x86emul: abstract out XCRn accesses

Use hooks, just like done for other special purpose registers.

This includes moving XCR0 checks from hvmemul_get_fpu() to the emulator
itself as well as adding support for XGETBV emulation.

For now fuzzer reads will obtain the real values (minus the fuzzing of
the hook pointer itself).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com> [tracing parts]