usb-device-manager: Fix reference counting getting devices

spice_usb_device_manager_device_to_bdev increments usb device
counter. The function is just used to check for filter but
than the counter is not decremented causing the object to be
leaked later (or invalid reference counter anyway).
Decrement the counter in any case.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Victor Toso <victortoso@redhat.com>
2 jobs for master in 3 minutes and 32 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #467167
fedora

00:03:32

passed #467168
windows

00:02:07