virsh: avoid uninitialized memory usage

Detected by valgrind, via Alex Jia.  Caused by imcomplete
copy-and-paste from vshWatchJob in commit 3b96a892.

* tools/virsh.c (cmdBlockPull): fix uninitialized memory usage.

* How to reproduce?
$ qemu-img create /var/lib/libvirt/images/test 1M
$ cat > /tmp/test.xml <<EOF
<domain type='qemu'>
  <name>test</name>
  <memory>219200</memory>
  <vcpu>1</vcpu>
  <os>
    <type arch='x86_64'>hvm</type>
    <boot dev='hd'/>
  </os>
  <devices>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/test'/>
      <target dev='vda' bus='virtio'/>
    </disk>
    <input type='mouse' bus='ps2'/>
    <graphics type='spice' autoport='yes' listen='0.0.0.0'/>
  </devices>
</domain>
EOF
$ virsh define /tmp/test.xml
$ valgrind -v virsh blockpull test /var/lib/libvirt/images/test --wait

actual result:

==10906== 1 errors in context 1 of 1:
==10906== Syscall param rt_sigaction(act->sa_flags) points to uninitialised byte(s)
==10906==    at 0x39CF80F5BE: __libc_sigaction (sigaction.c:67)
==10906==    by 0x43016C: cmdBlockPull (virsh.c:7638)
==10906==    by 0x4150D4: vshCommandRun (virsh.c:18574)
==10906==    by 0x425E73: main (virsh.c:20178)
==10906==  Address 0x7fefffae8 is on thread 1's stack

tools/virsh.c

@@ -7634,6 +7634,7 @@ cmdBlockPull(vshControl *ctl, const vshCmd *cmd)
 
         intCaught = 0;
         sig_action.sa_sigaction = vshCatchInt;
+        sig_action.sa_flags = SA_SIGINFO;
         sigemptyset(&sig_action.sa_mask);
         sigaction(SIGINT, &sig_action, &old_sig_action);