arm: virt: add PMU property to mach-virt machine type

CPU vPMU is now turned ON by default, but this feature wasn't introduced
until virt-2.7 machine type. To solve this problem, this patch adds a
PMU option in machine state, which is used to control CPU's vPMU status.
This PMU option is not exposed to command line and is turned off in
virt-2.6 machine type.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Wei Huang <wei@redhat.com>
Message-id: 1477463301-17175-3-git-send-email-wei@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
