target/arm: Swap PMU values before/after migrations

target/arm: Swap PMU values before/after migrations

Because of the PMU's design, many register accesses have side effects
which are inter-related, meaning that the normal method of saving CP
registers can result in inconsistent state. These side-effects are
largely handled in pmu_op_start/finish functions which can be called
before and after the state is saved/restored. By doing this and adding
raw read/write functions for the affected registers, we avoid
migration-related inconsistencies.

Signed-off-by: Aaron Lindsay <aclindsa@gmail.com>
Signed-off-by: Aaron Lindsay <aaron@os.amperecomputing.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20181211151945.29137-4-aaron@os.amperecomputing.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>