lxc: completely rework reference counting

lxc: completely rework reference counting

This patch follows the pattern used in qemu driver regarding
reference counting.
It changes lxcDomObjFromDomain() to ref the domain (using
virDomainObjListFindByUUIDRef()) and adds virDomainObjEndAPI() which
should be the only function in which the return value of
virObjectUnref() is checked.  This makes all reference counting
deterministic and makes the code a bit clearer.

Signed-off-by: Katerina Koukiou <k.koukiou@gmail.com>