HV:treewide:Update return type of function ffs64 and ffz64

To reduce type conversion in HV:
Update return type of function ffs64 and ffz64 as uint16;
For ffs64, when the input is zero, INVALID_BIT_INDEX is returned;
Update temporary variable type and return value check of caller
when it call ffs64 or ffz64;

Note: In the allocate_mem, there is no return value checking for
calling ffz64, this will be updated latter.

V1-->V2:
        INVALID_BIT_INDEX instead of INVALID_NUMBER
        Coding style fixing;
        INVALID_CPU_ID instead of INVALID_PCPU_ID or INVALID_VCPU_ID;
        "%hu" is used to print vcpu id (uint16_t);
        Add "U/UL" for constant value as needed.
V2-->V3:
        ffs64 return INVALID_BIT_INDEX directly when
        the input value is zero;
        Remove excess "%hu" updates.
V3-->V4:
        Clean up the comments of ffs64;
        Add "U" for constant value as needed.

Signed-off-by: Xiangyang Wu <xiangyang.wu@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>

hypervisor/include/lib/mem_mgt.h

@@ -8,7 +8,7 @@
 #define __MEM_MGT_H__
 
 /* Macros */
-#define BITMAP_WORD_SIZE         32
+#define BITMAP_WORD_SIZE         32U
 
 struct mem_pool {
 	void *start_addr;	/* Start Address of Memory Pool */