Monitor/QMP: Don't allow cont on bad VM state

Monitor/QMP: Don't allow cont on bad VM state

We have two states where issuing cont before system_reset can
cause problems: RSTATE_SHUTDOWN (when -no-shutdown is used) and
RSTATE_PANICKED (which only happens with kvm).

This commit fixes that by doing the following when state is
RSTATE_SHUTDOWN or RSTATE_PANICKED:

 1. returning an error to the user/client if cont is issued
 2. automatically transition to RSTATE_PAUSED during system_reset

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>