qemu: Warn when using vhost-user without shared memory
qemu: Warn when using vhost-user without shared memory

When user configures vhost-user interface and forgets to also configure
any shared memory, the search for the root cause of non-operational
interface might take unpleasantly long time.  Let's enhance user
experience by emitting a warning in the logs.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1266982

Signed-off-by: Martin Kletzander <e82f23e74d43760279ebcc505dac4ae70d5627d0@redhat.com>

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