The code that calls virCgroupSetCpuShares() and virCgroupGetCpuShares()
is repeated in 4 different places. Let's put it in a new
virCgroupSetupCpuShares() to avoid code repetition.

There's a reason of why we execute a Get in the same value we
just executed Set, explained in detail by commit 97814d8a.
Let's add a gist of the reasoning behind it as a comment in
this new function as well.
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>