x86/HVM: use x86emul_write_xcr()

x86/HVM: use x86emul_write_xcr()

...  instead of directly calling handle_xsetbv(), to make use of the
additional checking there.

Also don't call hvm_monitor_crX(XCR0, ...) for indexes other than zero
anymore.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>