virtualmonitor: Prevent crashing with a misbehaving virtual monitor device

While working on adding Virtual Monitor support to network/kdeconnect-ios>, I discovered that trying to enable Virtual Monitor output on the iOS device (from the host) would always crash kdeconnectd. This was almost certainly due to the fact that my implementation was incomplete, but I felt that a misbehaving device shouldn't be able to cause crashes in kdeconnectd, so I decided to try fixing it.

After fixing all the bugs I encountered, I can no longer cause kdeconnectd to crash. Each bug is fixed in a separate commit, and each commit message contains an explanation of that commit's bug and how it was fixed.