qemuDomainReboot: use fakeReboot=true only for acpi mode

qemuDomainReboot: use fakeReboot=true only for acpi mode

When acpi is used to reboot/shutdown qemu domain, qemu emits
SHUTDOWN event. Libvirt uses fakeReboot variable in order to
differentiate reboot or shutdown. fakeReboot value is reseted
to false after domain restart/reset.

When mode=agent is used to reboot qemu domain, qemu doesn't emit
SHUTDOWN event and libvirt doesn't reset fakeReboot value to false.
In this case next 'shutdown -h now' performs reboot. That's why
we don't need to set fakeReboot=true for mode=agent.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>