Respect boot_wait for VNC

[BytesGuy]

Currently the boot_wait command does not do anything, even though it is documented as "The time to wait after booting the initial virtual machine before typing the boot_command".

Currently the workaround I have been using is to add <wait90s> in the boot_command, which works but makes the config a little harder to follow.

It seems this was caused as the boot wait was not currently implemented fully, so this PR adds this functionality.

For example, setting boot_wait = "90s" will now produce:

==> tart-cli.vm: Starting the virtual machine...
==> tart-cli.vm: Waiting for the VNC server credentials from Tart...
==> tart-cli.vm: Retrieved VNC credentials, connecting...
==> tart-cli.vm: Connected to the VNC!
==> tart-cli.vm: Waiting 1m30s for VM to boot...
==> tart-cli.vm: Typing the commands over VNC...

This allows time for the VM to boot properly before VNC commands are issued, preventing unexpected behaviour.

[fkorotkov]

Thank you for noting! And thank you so much for the PR! Left two minor notes.