fix "Procedure is not pure assembler"

Misra C reqires assembly code should comply with
the rules list below:
  The assembly code's functionality should match the function's
name.If not,pls encapsulate the assembly code and give a suitable
name for describing the functionality.
V1->V2:
    1.remove the dead code
    2.update detail comment

V2->V3:
    1.replace the macro name with upper case.
    2.remove the typedef and rename the struct name
"_descriptor_table_" to "descriptor_table".

Tracked-On: #861
Signed-off-by: Huihuang Shi <huihuang.shi@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>

hypervisor/arch/x86/vtd.c

@@ -264,7 +264,7 @@ dmar_wait_completion(const struct dmar_drhd_rt *dmar_uint, uint32_t offset,
 		}
 		ASSERT(((rdtsc() - start) < CYCLES_PER_MS),
 			"DMAR OP Timeout!");
-		asm volatile ("pause" ::: "memory");
+		pause_cpu();
 	}
 }