Move QEMU caps validation of QEMU_CAPS_USB_HUB to a new function in
qemu_domain.c, qemuDomainDeviceDefValidateHub(). This function is
called by qemuDomainDeviceDefValidate() to validate the sound device
in domain define time.

qemuxml2xmltest.c was adjusted to add the now required caps for
domain definition.
Reviewed-by: NCole Robinson <crobinso@redhat.com>
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>