libxl: copy function for builtin types

libxl: copy function for builtin types

These functions will be used in later patch to deep-copy a structure.

Functions introduced:
 * libxl_string_list_copy
 * libxl_key_value_list_copy
 * libxl_hwcap_copy
 * libxl_mac_copy
 * libxl_cpuid_policy_list_copy
 * libxl_string_copy
 * libxl_bitmap_copy_alloc
 * libxl_ms_vm_genid_copy

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>