This prevents KVM guests from crashing that run iPXE on host CPUs
without "unrestricted_guest" support.

Once KVM gets the FXSAVE / FXRSTOR emulation feature (*), and the
feature becomes widely available to users, we can back out this change
from iPXE.

(*) Already in progress by Radim:

    [PATCH 0/2] KVM: x86: emulate fxsave and fxrstor

    https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1258895.html
    https://www.spinics.net/lists/kernel/msg2370327.html

Cc: Bandan Das <bsd@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Greg <rollenwiese@yahoo.com>
Cc: Michael Brown <mcb30@ipxe.org>
Cc: Michael Prokop <launchpad@michael-prokop.at>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Pickford <arch@netremedies.ca>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Ref: https://bugs.archlinux.org/task/50778
Ref: https://bugs.launchpad.net/qemu/+bug/1623276
Ref: https://bugzilla.proxmox.com/show_bug.cgi?id=1182
Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1356762
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>