hv: pirq: refactor vector allocation/free

This commit refactors vector allocation/free codes, two functions are
defined to help alloc/free vectors for an irq:
- uint32_t alloc_irq_vector(uint32_t irq)
  - alloc a free vector (0x20 ~ 0xDF), and bind it to irq, for legacy irqs
    and static mapped irqs, vector has been allocated and bind, so just
    check the mapping correctness;
  - return: valid vector on success, VECTOR_INVALID on failure.

- void free_irq_vector(uint32_t irq)
  - free vector allocated via alloc_irq_vector(), for legacy irqs and static
    mapped irqs, nothing need to do.

Signed-off-by: Yan, Like <like.yan@intel.com>
Acked-by: Anthony Xu <anthony.xu@intel.com>

hypervisor/arch/x86/ioapic.c

@@ -371,8 +371,8 @@ void setup_ioapic_irq(void)
 			 * for legacy irq, reserved vector and never free
 			 */
 			if (gsi < NR_LEGACY_IRQ) {
-				vr = irq_desc_alloc_vector(gsi);
-				if (vr > NR_MAX_VECTOR) {
+				vr = alloc_irq_vector(gsi);
+				if (vr == VECTOR_INVALID) {
 					pr_err("failed to alloc VR");
 					gsi++;
 					continue;