v4l2bufferpool: Validate that capture buffers were queued

v4l2bufferpool: Validate that capture buffers were queued

When the pool is started, we allocate and release buffer, expecting
the pool release-buffer handler to queue them. Though, as we rely
on release function, there is no direct way to detect that this
process didn't work.

To check this, validate that the number of queued buffer is the same
as the number of allocated buffers. This allow returning an error
when buffer importation was refused by the driver.

https://bugzilla.gnome.org/show_bug.cgi?id=583890