machine/memory encryption: Disable mem merge

machine/memory encryption: Disable mem merge

When a host is running with memory encryption, the memory isn't visible
to the host kernel; attempts to merge that memory are futile because
what it's really comparing is encrypted memory, usually encrypted
with different keys.

Automatically turn mem-merge off when memory encryption is specified.

https://bugzilla.redhat.com/show_bug.cgi?id=1796356

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20200130175046.85850-1-dgilbert@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>