feat: Connect to a detached running vm

[sudoAlphaX]

I confirm this feature has not been previously requested

• I have searched the issues and this feature has not previously been requested

Is your feature request related to a problem? Please describe.
It is difficult to connect to a running vm (started by qemu), when the window is terminated in cases like a compositor crash. In that case, the vm is still running. Trying to start the vm reports:

virtual-machine
 - Process:  Already running ./virtual-machine.conf as virtual-machine (<a pid of qemu-system-x86_64>)

Describe the solution you'd like
It will be helpful to connect to a detached virtual machine using a flag or just by running the connect command.

[lj3954]

Can you demonstrate this being possible with SDL and/or GTK displays? For SPICE displays, this is definitely possible and should be the default behaviour.

[flexiondotorg]

As far as I am aware, it is not possible to reattach QEMU to a running VM that was started using the SDL or GTK backends.

I have been considering changing quickemu to start VMs using SPICE by default, which would be one way to address this issue. There are other benefits to using SPICE, such as clipboard and USB sharing.