monitor: Add drift info to 'info jit'

Show in 'info jit' the current delay between the host clock
and the guest clock. In addition, print the maximum advance
and delay of the guest compared to the host.

Signed-off-by: Sebastian Tanase <sebastian.tanase@openwide.fr>
Tested-by: Camille Bégué <camille.begue@openwide.fr>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>