util: probe stub driver from within function that binds to stub driver

util: probe stub driver from within function that binds to stub driver

virPCIProbeStubDriver() and virPCIDeviceBindToStub() both have
very similar code that locally sets a driver name (based on
stubDriverType). These two functions are each also called in just one
place (virPCIDeviceDetach()), with just a small bit of validation code
in between.

To eliminate the "duplicated" code (which is going to be expanded
slightly in upcoming patches to support manually or automatically
picking a VFIO variant driver), this patch modifies
virPCIProbeStubDriver() to take the driver name as an argument
(rather than the virPCIDevice object), and calls it from within
virPCIDeviceBindToStub() (rather than from that function's caller),
using the driverName it has just figured out with the
now-not-duplicated code.

(NB: Since it could be used to probe *any* driver module, the name is
changed to virPCIProbeDriver()).

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>