devargs: introduce insert function

devargs: introduce insert function

Some buses will operate either in whitelist or blacklist mode.
This mode is currently passed down by the rte_eal_devargs_add function
with the devtype argument.

When inserting devices using the hotplug API, the implicit assumption is
that this device is being whitelisted, meaning that it is explicitly
requested by the application to be used. This can conflict with the
initial bus configuration.

While the rte_eal_devargs_add API is being deprecated soon, it cannot
be modified at the moment to accommodate this situation.
As such, this new experimental API offers a bare interface for inserting
rte_devargs without directly manipulating the global rte_devargs list.

This new function expects a fully-formed rte_devargs, previously parsed
and allocated.

It does not check whether the new rte_devargs is compatible with current
bus configuration, but will replace any eventual existing one for the same
device, allowing the hotplug operation to proceed. i.e. a previously
blacklisted device can be redefined as being whitelisted.

Signed-off-by: Gaetan Rivet <gaetan.rivet@6wind.com>