feat(interrupts): allow interrupt to be enabled or disabled in runtime

This patch extends the paravirtualized interface introduced in earlier
patches to either enable or disable a secure physical interrupt.

Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
Change-Id: I86250d8e4788feae4d2b9898567c640aa54b710f
diff --git a/src/load.c b/src/load.c
index 1c1794d..ae97a2d 100644
--- a/src/load.c
+++ b/src/load.c
@@ -151,6 +151,7 @@
 	}
 
 	interrupt_desc_set_valid(int_desc, true);
+	interrupt_desc_set_enabled(int_desc, true);
 }
 
 /**
@@ -184,6 +185,7 @@
 			interrupt = dev_region.interrupts[j];
 			infer_interrupt(interrupt, &int_desc);
 			vm_locked.vm->interrupt_desc[k] = int_desc;
+			assert(int_desc.enabled);
 
 			/*
 			 * Configure the physical interrupts allocated for this