Make `quiesce` a parameter of the guest

[lafrech]

vmbackup snapshots using the quiesce option.

The guest must have guest agent installed, otherwise, the snapshotting will fail.

Possible solutions:

1/ Try with quiesce, catch error, then retry without it.

2/ Let quiesce be a parameter taken from the config file. The parameter line could be:

vm_names = machine1:quiesce,other_option machine2 machine3

Machines without options would be written without trailing colon for retro-compatibility and simplicity.

Solution 2 allows other options to be passed as well.

Applying both 1 and 2 allows the user to decide which machine uses quiesce and falls back on "no quiesce" when quiesce was indicated but the guest agent isn't working.