qemu_capabilities: Do not report USB as subsystem type if it is not available

qemu_capabilities: Do not report USB as subsystem type if it is not available

libvirt currently always reports that USB is available as a bus subsystem
type when running "virsh domcapabilities". However, this is not always
true, for example the qemu-system-s390x binary normally never has support
for USB. Thus we should only report that USB is available if there is
also a USB host controller available where we can attach USB devices.

Reported-by: Sebastian Mitterle <smitterl@redhat.com>
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1759849
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>