iotests: add support for capturing and matching QMP events

iotests: add support for capturing and matching QMP events

When using the _launch_qemu and _send_qemu_cmd functions from
common.qemu, any QMP events get mixed in with the output from
the commands and responses.

This makes it difficult to write a test case as the ordering
of events in the output is not stable.

This introduces a variable 'capture_events' which can be set
to a list of event names. Any events listed in this variable
will not be printed, instead collected in the $QEMU_EVENTS
environment variable.

A new '_wait_event' function can be invoked to collect events
at a fixed point in time. The function will first pull events
cached in $QEMU_EVENTS variable, and if none are found, will
then read more from QMP.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210204124834.774401-11-berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>