xen: Fix xenpv machine initialisation

When using QEMU for Xen PV guest, QEMU abort with:
xen-common.c:118:xen_init: Object 0x7f2b8325dcb0 is not an instance of type generic-pc-machine

This is because the machine 'xenpv' also use accel=xen. Moving the code
to xen_hvm_init() fix the issue.

This fix 021746c1.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>