qemu: support xen hvm direct kernel boot

qemu: support xen hvm direct kernel boot

qemu side patch to support xen HVM direct kernel boot:
if -kernel exists, calls xen_load_linux(), which will read kernel/initrd
and add a linuxboot.bin or multiboot.bin option rom. The
linuxboot.bin/multiboot.bin will load kernel/initrd and jump to execute
kernel directly. It's working when xen uses seabios.

During this work, found the 'kvmvapic' is in option_rom list, it should
not be there in xen case. Set s->vapic_control = 0 in xen_apic_realize()
to handle that.

Signed-off-by: Chunyan Liu <cyliu@suse.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>