Mer: Properly check if VM is running headless on exit

Signed-off-by: Martin Kampas <martin.kampas@jolla.com>
Mer: Check for VM existence when connection requested

Reintroducing previously available (similar) functionality which I
unintentionally removed with 13d52e8 (Mer: Do not lose VM connection
state)

Signed-off-by: Martin Kampas <martin.kampas@jolla.com>
Mer: Do not allow to add emulator w/o VM

Signed-off-by: Martin Kampas <martin.kampas@jolla.com>
Mer: Do not leak MerSdk instances during configuration

Signed-off-by: Martin Kampas <martin.kampas@jolla.com>
Mer: Only offer unused VMs for configuration

Signed-off-by: Martin Kampas <martin.kampas@jolla.com>
Mer: Fix deadlock on auto reseting VM after connection error

Also fix related debug message.

Signed-off-by: Martin Kampas <martin.kampas@jolla.com>
Mer: Improve user dialog on auto reseting VM after error

Do not ask "Close VM?" when "Connect" is requested after error.

Signed-off-by: Martin Kampas <martin.kampas@jolla.com>
Merge pull request #20 from martyone/next

VM related