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>