While trying to build with -Os couple of compile errors showed
up.

conf/domain_conf.c: In function 'virDomainChrRemove':
conf/domain_conf.c:13666:24: error: 'ret' may be used uninitialized in this function [-Werror=maybe-uninitialized]
     virDomainChrDefPtr ret, **arrPtr = NULL;
                        ^
Compiler fails to see that @ret is used only if set in the loop,
but whatever, there's no harm in initializing the variable.

In vboxAttachDrivesNew and _vboxAttachDrivesOld compiler thinks
that @rc may be used uninitialized. Well, not directly, but maybe
after some optimization. Yet again, no harm in initializing a
variable.

In file included from ./util/virthread.h:26:0,
                 from ./datatypes.h:28,
                 from vbox/vbox_tmpl.c:43,
                 from vbox/vbox_V3_1.c:37:
vbox/vbox_tmpl.c: In function '_vboxAttachDrivesOld':
./util/virerror.h:181:5: error: 'rc' may be used uninitialized in this function [-Werror=maybe-uninitialized]
     virReportErrorHelper(VIR_FROM_THIS, code, __FILE__,              \
     ^
In file included from vbox/vbox_V3_1.c:37:0:
vbox/vbox_tmpl.c:1041:14: note: 'rc' was declared here
     nsresult rc;
              ^
Yet again, one uninitialized variable:

qemu/qemu_driver.c: In function 'qemuDomainBlockCommit':
qemu/qemu_driver.c:17194:9: error: 'baseSource' may be used uninitialized in this function [-Werror=maybe-uninitialized]
         qemuDomainPrepareDiskChainElement(driver, vm, baseSource,
         ^

And another one:

storage/storage_backend_logical.c: In function 'virStorageBackendLogicalMatchPoolSource.isra.2':
storage/storage_backend_logical.c:618:33: error: 'thisSource' may be used uninitialized in this function [-Werror=maybe-uninitialized]
                       thisSource->devices[j].path))
                                 ^
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
(cherry picked from commit bde6e002)