vl.c: error out if -mem-path is used together with -M memory-backend

the former is not actually used by explicit backend, so instead of
silently ignoring the option in non valid context, exit with error.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20200409134133.11339-1-imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
1 file changed