KVM: arm64: Alloc simple_buffer_page using pKVM hyp allocator

In protected mode, transition the allocation of the simple_ring_buffer
structures from the host to the hypervisor using the new pKVM heap
allocator.

Previously, the host allocated and donated a contiguous backing memory
for these structures. In pKVM the hypervisor can now allocate them
dynamically.

Use the pkvm_call_hyp_req() wrapper in the host to invoke
__tracing_load, which automatically handles any top-up requests if the
hypervisor runs out of heap memory during allocation.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>