There is duplicated code between virt drivers that needs to
be moved to avoid code repetition. In the case of duplicated
code between lxc_cgroup.c and qemu_cgroup.c a common place
would be utils/vircgroup.c. The problem is that this would
introduce /conf related definitions that shouldn't be imported
to vircgroup.c, which is supposed to be a place for utilitary
cgroups functions only. And syntax-check would forbid it anyway
due to cross-directory includes being used.

An alternative would be to overload domain_conf.c, which already
contains all the definitions required. But that file is already
crowded with XML handling code and we wouldn't do any favors to
it by putting more utilitary, non-XML parsing/formatting code
there.

In [1], Cole suggested a 'domain_cgroup' file to host common code
between lxc_cgroup and qemu_cgroup, and Daniel suggested a
'src/hypervisor' dir to host these type of files. This patch
introduces src/hypervisor/domain_cgroup.c and, to get started,
introduces a new virDomainCgroupSetupBlkio() function to host shared
code between virLXCCgroupSetupBlkioTune() and qemuSetupBlkioCgroup().

[1] https://www.redhat.com/archives/libvir-list/2019-December/msg00817.htmlSigned-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>