build: avoid uninitialized use warning

build: avoid uninitialized use warning

Otherwise, with new enough gcc compiling at -O2, the build fails with:
../../src/conf/domain_conf.c: In function ‘virDomainDeviceDefPostParse’:
../../src/conf/domain_conf.c:2821:29: error: ‘cnt’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
             for (i = 0; i < *cnt; i++) {
                             ^
../../src/conf/domain_conf.c:2795:20: note: ‘cnt’ was declared here
         size_t i, *cnt;
                    ^
../../src/conf/domain_conf.c:2794:30: error: ‘arrPtr’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
         virDomainChrDefPtr **arrPtr;
                              ^

* src/conf/domain_conf.c (virDomainChrGetDomainPtrs): Always
assign into output parameters.

Signed-off-by: Eric Blake <eblake@redhat.com>