hv_balloon: Reorganize the probe function
Move the code that negotiates with the host to a new function
balloon_connect_vsp() and improve the error handling.
This makes the code more readable and paves the way for the
support of hibernation in future.
Makes no real logic change here.
Signed-off-by: Dexuan Cui <email@example.com>
Reviewed-by: Michael Kelley <firstname.lastname@example.org>
Signed-off-by: Sasha Levin <email@example.com>