lxcDomainMergeBlkioDevice() and qemuDomainMergeBlkioDevice()
are the same functions. This duplicated code can't be put in
the existing domain_cgroup.c since it's not cgroup related.

This patch introduces a new src/hypervisor/domain_driver.c to
host this more generic code that can be shared between virt
drivers. This new file is then used to create a new helper
called virDomainDeivceMergeBlkioDevice() to eliminate the code
repetition mentioned above. Callers in LXC and QEMU files
were updated.

This change is a preliminary step for more code reduction of
cgroup related code inside lxcDomainSetBlkioParameters() and
qemuDomainSetBlkioParameters().
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>