kvm powerpc/book3s-apiv2: Add kunit tests for Hostwide GSB elements

kvm powerpc/book3s-apiv2: Add kunit tests for Hostwide GSB elements

Update 'test-guest-state-buffer.c' to add two new KUNIT test cases for
validating correctness of changes to Guest-state-buffer management
infrastructure for adding support for Hostwide GSB elements.

The newly introduced test test_gs_hostwide_msg() checks if the Hostwide
elements can be set and parsed from a Guest-state-buffer. The second kunit
test test_gs_hostwide_counters() checks if the Hostwide GSB elements can be
send to the L0-PowerVM hypervisor via the H_GUEST_SET_STATE hcall and
ensures that the returned guest-state-buffer has all the 5 Hostwide stat
counters present.

Below is the KATP test report with the newly added KUNIT tests:

KTAP version 1
    # Subtest: guest_state_buffer_test
    # module: test_guest_state_buffer
    1..7
    ok 1 test_creating_buffer
    ok 2 test_adding_element
    ok 3 test_gs_bitmap
    ok 4 test_gs_parsing
    ok 5 test_gs_msg
    ok 6 test_gs_hostwide_msg
    # test_gs_hostwide_counters: Guest Heap Size=0 bytes
    # test_gs_hostwide_counters: Guest Heap Size Max=10995367936 bytes
    # test_gs_hostwide_counters: Guest Page-table Size=2178304 bytes
    # test_gs_hostwide_counters: Guest Page-table Size Max=2147483648 bytes
    # test_gs_hostwide_counters: Guest Page-table Reclaim Size=0 bytes
    ok 7 test_gs_hostwide_counters
 # guest_state_buffer_test: pass:7 fail:0 skip:0 total:7
 # Totals: pass:7 fail:0 skip:0 total:7
 ok 1 guest_state_buffer_test

Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250416162740.93143-4-vaibhav@linux.ibm.com