blkdebug: store list of active rules

This prepares for the next patch, where some active rules may actually
not trigger depending on input to readv/writev.  Store the active rules
in a SIMPLEQ (so that it can be emptied easily with QSIMPLEQ_INIT), and
fetch the errno/once/immediately arguments from there.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>