Consolidate "cont" into qemudMonitorSendCont()

Consolidate "cont" into qemudMonitorSendCont()

The interface allows qemudMonitorSendCont() to report errors that are
not overridden by its callers.

Also fix a potential infinite loop in qemuDomainCoreDump() if sending
cont repeatedly fails.

* src/qemu_driver.c (qemudMonitorSendCont): New function.
  (qemudAutostartConfigs): Reset error before each call to
  qemudStartVMDaemon().
  (qemudInitCpus, qemudDomainResume, qemudDomainCoreDump,
  qemudDomainRestore, qemudDomainMigratePerform,
  qemudDomainMigrateFinish2): Use qemudMonitorSendCont().