vm/vmimpl: add -O flag to scp for legacy protocol compatibility

[guilherme-n-l]

Minimal dropbear builds (without SFTP subsystem) don't work with syzkaller's scp
file transfers. Since OpenSSH 9.0, scp defaults to SFTP protocol. When the VM
runs dropbear without SFTP support, transfers fail silently.

It would be useful to have a configuration option (e.g. scp_legacy_protocol in
the VM config) that adds -O to scp calls, forcing the legacy SCP protocol.
This should not be enabled by default, only when the target VM lacks SFTP support.

Affected: vm/vmimpl/util.go, scpArgs function.