If users only specified vendor&product (the common case) then parsing
the xml via virDomainHostdevSubsysUSBDefParseXML would only set these.
Bus and Device would much later be added when the devices are prepared
to be added.

Due to that a hot-add of a usb hostdev works as the device is prepared
and virt-aa-helper processes the new internal xml. But on an initial
guest start at the time virt-aa-helper renders the apparmor rules the
bus/device id's are not set yet:

p ctl->def->hostdevs[0]->source.subsys.u.usb
$12 = {autoAddress = false, bus = 0, device = 0, vendor = 1921, product
= 21888}

That causes rules to be wrong:
  "/dev/bus/usb/000/000" rw,

The fix calls virHostdevFindUSBDevice after reading the XML from
virt-aa-helper to only add apparmor rules for devices that could be found
and now are fully known to be able to write the rule correctly.

It uncondtionally sets virHostdevFindUSBDevice mandatory attribute as
adding an apparmor rule for a device not found makes no sense no matter
what startup policy it has set.
Signed-off-by: NChristian Ehrhardt <christian.ehrhardt@canonical.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>