conf: remove duplicated typedefs for virDomainSevDef

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>