-xen-attach is needed for pvh boot with qemu-xen

-xen-attach is needed for pvh boot with qemu-xen

Currently the boot of a pvh guest using the qemu-xen device model fails
with the error
xen emulation not implemented (yet)
in the qemu-dm log file. This patch adds the missing -xen-attach
argument.

V2: Use  b_info->type != LIBXL_DOMAIN_TYPE_HVM    instead of
 	(b_info->type == LIBXL_DOMAIN_TYPE_PV) ||
 	(b_info->type == LIBXL_DOMAIN_TYPE_PVH)
as recommended by Roger Pau Monné.

Signed-off-by: Michael Young <m.a.young@durham.ac.uk>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
(cherry picked from commit 6f1979c8e4184f1f2b24b860e30d3b037b2e7f05)