conf: don't redefine virDomainCapsDeviceHostdev

conf: don't redefine virDomainCapsDeviceHostdev

Commit 5ed235c6 added unnecessary redifinition of
virDomainCapsDeviceHostdev in conf/domain_capabilities.h. This breaks
build with clang 3.4:

In file included from conf/domain_capabilities.c:25:
conf/domain_capabilities.h:88:44: error: redefinition of typedef
'virDomainCapsDeviceHostdev' is a C11 feature
[-Werror,-Wtypedef-redefinition]
typedef struct _virDomainCapsDeviceHostdev virDomainCapsDeviceHostdev;
                                           ^
conf/domain_capabilities.h:86:44: note: previous definition is here
typedef struct _virDomainCapsDeviceHostdev virDomainCapsDeviceHostdev;

So drop one of those.