The QEMU driver can take an optional `graceful_shutdown` configuration
which will create a Unix socket to send ACPI shutdown signal to the VM.

Unix sockets have a hard length limit and the driver implementation
assumed that QEMU versions 2.10.1 were able to handle longer paths. This
is not correct, the linked QEMU fix only changed the behaviour from
silently truncating longer socket paths to throwing an error.

By validating the socket path before starting the QEMU machine we can
provide users a more actionable and meaningful error message, and by
using a shorter socket file name we leave a bit more room for
user-defined values in the path, such as the task name.

The maximum length allowed is also platform-dependant, so validation
needs to be different for each OS.