vm: overhaul

VM infrastructure currently has several problems:
 - Config struct is complete mess with a superset of params for all VM types
 - verification of Config is mess spread across several places
 - there is no place where VM code could do global initialization
   like creating GCE connection, uploading GCE image to GCS,
   matching adb devices with consoles, etc
 - it hard to add private VM implementations
   such impl would need to add code to config package
   which would lead to constant merge conflicts
 - interface for VM implementation is mixed with interface for VM users
   this does not allow to provide best interface for both of them
 - there is no way to add common code for all VM implementations

This change solves these problems by:
 - splitting VM interface for users (vm package) and VM interface
   for VM implementations (vmimpl pacakge), this in turn allows
   to add common code
 - adding Pool concept that allows to do global initialization
   and config checking at the right time
 - decoupling manager config from VM-specific config
   each VM type now defines own config

Note: manager configs need to be changed after this change:
VM-specific parts are moved to own "vm" subobject.

Note: this change also drops "local" VM type.
Its story was long unclear and there is now syz-stress which solves the same problem.

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