vl: make machine type deprecation a warning

error_report should generally be followed by a failure; if we can proceed
anyway, that is just a warning and should be communicated properly to
the user with warn_report.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20220511175043.27327-1-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>