conf: Introduce @guestReset to hostdev's <source/>

conf: Introduce @guestReset to hostdev's <source/>

Some USB devices have a buggy firmware that either crashes on
device reset, or make the device unusable in some other way.
Fortunately, QEMU offers a way to skip device reset either
completely, or if device is not initialized yet. Expose this
ability to users under:

    <hostdev mode='subsystem' type='usb'>
      <source guestReset='off'/>
    </hostdev>

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>