xen: simplify xen_enabled

No need for preprocessor conditionals in xen_enabled: xen_allowed is
always defined.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
CC: qemu-stable@nongnu.org
(cherry picked from commit 49fa9881b2)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

include/hw/xen/xen.h

@@ -25,11 +25,7 @@ extern bool xen_allowed;
 
 static inline bool xen_enabled(void)
 {
-#if defined(CONFIG_XEN_BACKEND) && defined(CONFIG_XEN)
     return xen_allowed;
-#else
-    return 0;
-#endif
 }
 
 int xen_pci_slot_get_pirq(PCIDevice *pci_dev, int irq_num);