domain_capabilities: Don't report machine type for bhyve

For some drivers the domain's machine type makes no sense. They
just don't use it. A great example is bhyve driver. Therefore it
makes very less sense to report machine in domain capabilities
XML.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

docs/formatdomaincaps.html.in

@@ -70,7 +70,8 @@
 
       <dt><code>machine</code></dt>
       <dd>The domain's <a href="formatdomain.html#elementsOSBIOS">machine
-          type</a>.</dd>
+          type</a>. Since not every hypervisor has a sense of machine types
+          this element might be omitted in such drivers.</dd>
 
       <dt><code>arch</code></dt>
       <dd>The domain's <a href="formatdomain.html#elementsOSBIOS">

src/conf/domain_capabilities.c

@@ -527,7 +527,8 @@ virDomainCapsFormatInternal(virBufferPtr buf,
 
     virBufferEscapeString(buf, "<path>%s</path>\n", caps->path);
     virBufferAsprintf(buf, "<domain>%s</domain>\n", virttype_str);
-    virBufferAsprintf(buf, "<machine>%s</machine>\n", caps->machine);
+    if (caps->machine)
+        virBufferAsprintf(buf, "<machine>%s</machine>\n", caps->machine);
     virBufferAsprintf(buf, "<arch>%s</arch>\n", arch_str);
 
     if (caps->maxvcpus)