This way we don't need separate admin.vm.Clone call, which is tricky to
handler properly with policy.
A VM may not have access to all the properties and other metadata, so
add ignore_errors argument, for best-effort approach (copy what is
possible). In any case, failure of cloning VM data fails the whole
operation.
When operation fails, VM is removed.
While at it, allow to specify alternative VM class - this allows
morphing one VM into another (for example AppVM -> StandaloneVM).
Adjust qvm-clone tool and tests accordingly.
QubesOS/qubes-issues#2622
Enable 'qrexec' VM feature to wait for qrexec initialization - it is
required to call qubes.PostInstall service. If VM start fails, assume
there is no qrexec and drop that feature.
Since we use qubes.VMShell service now and send requested command on its
stdin, we need to terminate that shell after requested command -
otherwise the service will not terminate automatically waiting for
further input (next commands).
qubesd_call expect "bytes" type. Additionally serialize false value as
empty string (which is treated by python as false value), because
otherwise would be serialized to (non-empty) string, which is true value
in python.
