xen: pvhvm: allow user to request no emulated device unplug
this allows the user to disable pvhvm and revert to emulated devices
in case of a system misconfiguration (e.g. initramfs with only
emulated drivers in it).
Signed-off-by: Ian Campbell <ian.campbell@citrix.com> Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com> Acked-by: Stefano Stabellini <Stefano.Stabellini@eu.citrix.com>