domain_capabilities: Use virXMLFormatElement() in FORMAT_PROLOGUE and FORMAT_EPILOGUE...

domain_capabilities: Use virXMLFormatElement() in FORMAT_PROLOGUE and FORMAT_EPILOGUE macros

Domain capabilities XML is formatted (mostly) using
FORMAT_PROLOGUE and FORMAT_EPILOGUE macros. These format opening
and closing stanzas for given element. The FORMAT_PROLOGUE macro
even tries to be clever and format element onto one line (if the
element isn't supported), but that's not enough. Fortunately, we
have virXMLFormatElement() which formats elements properly, so
let's switch macros into using that.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>