Add -defaults option

This option is the logical opposite of the -nodefaults operation,
and force the use default devices.

Future changeset will change the default to not create default
devices.
Provide --disable-default-devices configuration option.

This option changes the standard configuration such that
default devices are not created (by default) when QEMU
starts.

Default devices can be created by passing -defaults on the command
line.
monitor: Ensure protocol events are initialised.

Currently if there is no monitor created then the protocol event
data structures are not correctly initialised.

This makes initialisation of the monitor module explicit and
separate from adding actual monitor objects.