Implement simple custom_config

[lj3954]

Only image types img & iso are supported by quickemu, so throw an error if the file/url doesn't end in one of those. Quickget shouldn't require the user to specify the OS, so instead attempt to determine it from the file (vast majority will be Linux-based anyways). make_vm_config had to be slightly modified to allow for the possibility of full paths to image files.

[zen0bit]

I like it but, just keep same structure also for custom VM

Only question is in "local mode". If image should be moved or copied from previous location

I am for moving (saving space)

PS: I will add qcow2 files support later...

[zen0bit]

Bit modified your create_config function in my PR #1107 Check if I messed something...

[flexiondotorg]

CLosed via https://github.com/quickemu-project/quickemu/pull/1143