powerpc/kvm-hv-pmu: Implement GSB message-ops for hostwide counters

Implement and setup necessary structures to send a prepolulated
Guest-State-Buffer(GSB) requesting hostwide counters to L0-PowerVM and have
the returned GSB holding the values of these counters parsed. This is done
via existing GSB implementation and with the newly added support of
Hostwide elements in GSB.

The request to L0-PowerVM to return Hostwide counters is done using a
pre-allocated GSB named 'gsb_l0_stats'. To be able to populate this GSB
with the needed Guest-State-Elements (GSIDs) a instance of 'struct
kvmppc_gs_msg' named 'gsm_l0_stats' is introduced. The 'gsm_l0_stats' is
tied to an instance of 'struct kvmppc_gs_msg_ops' named  'gsb_ops_l0_stats'
which holds various callbacks to be compute the size ( hostwide_get_size()
), populate the GSB ( hostwide_fill_info() ) and
refresh ( hostwide_refresh_info() ) the contents of
'l0_stats' that holds the Hostwide counters returned from L0-PowerVM.

To protect these structures from simultaneous access a spinlock
'lock_l0_stats' has been introduced. The allocation and initialization of
the above structures is done in newly introduced kvmppc_init_hostwide() and
similarly the cleanup is performed in newly introduced
kvmppc_cleanup_hostwide().

Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250416162740.93143-6-vaibhav@linux.ibm.com

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