Dynamically allocate the vrings' DMA when the remote processor
is about to be powered on (i.e. when ->find_vqs() is invoked),
and release them as soon as it is powered off (i.e. when ->del_vqs()
is invoked).

The obvious and immediate benefit is better memory utilization, since
memory for the vrings is now only allocated when the relevant remote
processor is used.

Additionally, this approach also makes recovery of a (crashing)
remote processor easier: one just needs to remove the relevant
vdevs, and the entire vrings cleanup takes place automagically.

Tested-by: Fernando Guzman Lugo <fernando.lugo@ti.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>