[mounts/windows] Mounting a native mount during instance start is very slow #2934
Description
When having a native mount defined during instance start, it is very slow to finish mounting. This is due to https://github.com/canonical/multipass-private/pull/503/commits where PowerShell is now a member of SmbMountHandler and when starting an instance, it runs a different thread and thus when powershell->run() is called, Qt breaks since it's running in a different thread than the one it was created in.
I think the easiest thing is convert back to calling PowerShell::exec() in place which instantiates the object in the thread that is calling it. The other option is the convert all of these PowerShell calls to a proper Windows API.