conf: remove duplicated typedefs for virDomainSevDef
    
    The typedefs were present twice in the header file which causes failures
    with some compilers, eg FreeBSD 10 CLang:
    
    ../../src/conf/domain_conf.h:2330:33: error: redefinition of typedef 'virDomainSevDef' is a C11 feature
    +[-Werror,-Wtypedef-redefinition]
    typedef struct _virDomainSevDef virDomainSevDef;
                                    ^
    ../../src/conf/domain_conf.h:145:33: note: previous definition is here
    typedef struct _virDomainSevDef virDomainSevDef;
                                    ^
    
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>