qemu_conf.c: introduce qemuAddRemoveSharedDiskInternal

qemu_conf.c: introduce qemuAddRemoveSharedDiskInternal

Following the same idea of avoid code repetition from the
previous patch, this commit introduces a new function that
aggregates the functions of qemuAddSharedDisk() and
qemuRemoveSharedDisk() into a single place, using a flag to
switch between add/remove operations.

Both qemuAddSharedDisk() and qemuRemoveSharedDisk() are
public, so keep them around to avoid changing other files
due to an internal qemu_conf.c refactory.

No functional change was made.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>