kvmtool: Handle exit reason KVM_EXIT_SYSTEM_EVENT

The KVM_EXIT_SYSTEM_EVENT exit reason was added to define
architecture independent system-wide events for a Guest.

Currently, it is used by in-kernel PSCI-0.2 emulation of
KVM ARM/ARM64 to inform user space about PSCI SYSTEM_OFF
or PSCI SYSTEM_RESET request.

For now, we simply treat all system-wide guest events as
shutdown request in KVMTOOL.

Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Signed-off-by: Anup Patel <anup.patel@linaro.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
[will: removed useless prints]
Signed-off-by: Will Deacon <will.deacon@arm.com>