qtest: add clock management

qtest: add clock management

This patch combines qtest and -icount together to turn the vm_clock
into a source that can be fully managed by the client.  To this end new
commands clock_step and clock_set are added.  Hooking them with libqtest
is left as an exercise to the reader.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>