arm: Add an option to turn on/off vPMU support

arm: Add an option to turn on/off vPMU support

This patch adds a pmu=[on/off] option to enable/disable vPMU support
in guest vCPU. It allows virt tools, such as libvirt, to determine the
exsitence of vPMU and configure it. Note this option is only available
for cortex-a57/cortex-53/ host CPUs, but unavailable on ARMv7 and other
processors. Also even though "pmu=" option is available for TCG mode,
setting it doesn't turn PMU on.

Signed-off-by: Wei Huang <[email protected]>
Reviewed-by: Andrew Jones <[email protected]>
Message-id: 1477463301[email protected]
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>