hw/i386/vmport: Assert vmport initialized before registering commands

hw/i386/vmport: Assert vmport initialized before registering commands

vmport_register() is also called from other modules such as vmmouse.
Therefore, these modules rely that vmport is realized before those call
sites. If this is violated, vmport_register() will NULL-deref.

To make such issues easier to debug, assert in vmport_register() that
vmport is already realized.

Reviewed-by: Nikita Leshenko <nikita.leshchenko@oracle.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Message-Id: <20200312165431.82118-17-liran.alon@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>