make: fix help message reference to bogus V=0 variable

The make rules for building QEMU are mostly silent by default. They can
be made verbose by setting the variable V=1. The default state does not
however correspond to a V=0 setting - $(V) must be undefined / empty to
get the default quiet build.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20180123164718.12714-3-berrange@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>