qemu: don't require is_kvm for vhost-net support

This was discussed in:

  https://www.redhat.com/archives/libvir-list/2011-May/msg01370.html

The capabilities code only sets the flag to allow use of vhost-net if
kvm is detected (set if the help string contains "(qemu-kvm-" or
"(kvm-"), but actually vhost-net is available in some qemu builds that
don't have kvm in their name, so just checking for ",vhost=" is enough.

src/qemu/qemu_capabilities.c

@@ -982,7 +982,7 @@ qemuCapsComputeCmdFlags(const char *help,
     if (is_kvm && (version >= 10000 || kvm_version >= 74))
         qemuCapsSet(flags, QEMU_CAPS_VNET_HDR);
 
-    if (is_kvm && strstr(help, ",vhost=")) {
+    if (strstr(help, ",vhost=")) {
         qemuCapsSet(flags, QEMU_CAPS_VNET_HOST);
     }