xen-pciback: fix INTERRUPT_TYPE_* defines

xen_pcibk_get_interrupt_type() assumes INTERRUPT_TYPE_NONE being 0
(initialize ret to 0 and return as INTERRUPT_TYPE_NONE).
Fix the definition to make INTERRUPT_TYPE_NONE really 0, and also shift
other values to not leave holes.
But also, do not assume INTERRUPT_TYPE_NONE being 0 anymore to avoid
similar confusions in the future.

Fixes: 476878e4 ("xen-pciback: optionally allow interrupt enable flag writes")
Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>