add cpu_test_interrupt()/cpu_set_interrupt() helpers and use them tree wide

add cpu_test_interrupt()/cpu_set_interrupt() helpers and use them tree wide

The helpers form load-acquire/store-release pair and ensure
that appropriate barriers are in place in case checks happen
outside of BQL.

Use them to replace open-coded checkers/setters across the code,
to make sure that barriers are not missed.  Helpers also make code a
bit more readable.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Jason J. Herne <jjherne@linux.ibm.com>
Link: https://lore.kernel.org/r/20250821155603.2422553-1-imammedo@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>