vl.c: Output error on invalid machine type

Output error message using qemu's error_report() function when user
provides the invalid machine type on the command line. This also saves
time to find what issue is when you downgrade from one version of qemu
to another that doesn't support required machine type yet (the version
user downgraded to have to have this patch applied too, of course).

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
[Replace printf with error_printf, suggested by Markus Armbruster. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
