Deleted (trashed) instances are not persistified

[ricab]

$ sudo ./bin/multipassd 
^Z
[3]+  Stopped                 sudo ./bin/multipassd
 $ bg
[3]+ sudo ./bin/multipassd &
 $ multipass list
Name                    State             IPv4             Release
relative-rockhopper     STOPPED           --               Ubuntu 16.04 LTS
sleek-cobra             STOPPED           --               Ubuntu 16.04 LTS
admissible-mudskipper   STOPPED           --               Ubuntu 16.04 LTS
 $ multipass delete --all
 $ multipass list
Name                    State             IPv4             Release
admissible-mudskipper   DELETED           --               Not Available
relative-rockhopper     DELETED           --               Not Available
sleek-cobra             DELETED           --               Not Available
 $ fg
sudo ./bin/multipassd
^C[warning] [Qt] "No such interface 'org.freedesktop.DBus.Properties' on object at path /org/freedesktop/NetworkManager/ActiveConnection/29"
[...]
 $ echo "multipassd just quit, let's restart..."
multipassd just quit, let's restart...
 $ sudo ./bin/multipassd 
^Z
[3]+  Stopped                 sudo ./bin/multipassd
 $ bg
[3]+ sudo ./bin/multipassd &
 $ multipass list
Name                    State             IPv4             Release
relative-rockhopper     STOPPED           --               Ubuntu 16.04 LTS
sleek-cobra             STOPPED           --               Ubuntu 16.04 LTS
admissible-mudskipper   STOPPED           --               Ubuntu 16.04 LTS

[ricab]

We currently don't have delete as part of internal states, even though we treat it as a state for many purposes, including user interface. Two important implications:

1. We need special code branches to deal with "deleted"
2. We don't persistify deleted instances (this issue)

I am therefore adding the corresponding enum entry to deal with both points. Item 1 above will be mitigated but not completely removed, as deleted (trashed) VMs are still kept in a separate collection.

[ricab]

The plan changed after a bit of discussion. Since we want to be able to recover instances to different states in the future (see #549), we will eventually need to persistify an additional piece of information.

The code already stores the deleted "state" independently, freeing the state attribute to store what state to go back to on recover (or, equivalently, the state before delete). It is easier to keep that and just store an additional flag indicating whether the instance had been deleted.