Require host directory mounts to use absolute path in GUI

**Describe the bug**
In the Multipass GUI, you can enter a host directory mount of `..`, which resolves to the parent directory where the daemon is running. The CLI, on the other hand, converts the host directory into an absolute path before sending it to the daemon. Since the GUI doesn't make it clear what the current directory is, we should just require the path to be absolute when the user enters it.

**To Reproduce**
How, and what happened?
1. Start the multipass GUI
2. Go to the instance / Details / Mounts / Configure / Add Mount
3. Enter `..` for a host directory
4. Enter `/home/ubuntu/blah` for the guest directory
5. Click Add
6. It works(?!)

**Expected behavior**
A UI indication that the host directory is invalid and that the user should enter an absolute path.