qemuDomainGetVcpusFlags: Initialize ncpuinfo

Currently, there's a path to use the ncpuinfo variable uninitialized,
which leads to a compiler warning:

    qemu/qemu_driver.c: In function 'qemuDomainGetVcpusFlags':
    qemu/qemu_driver.c:4573:9: error: 'ncpuinfo' may be used
    uninitialized in this function [-Werror=maybe-uninitialized]
             for (i = 0; i < ncpuinfo; i++) {
             ^

src/qemu/qemu_driver.c

@@ -4512,7 +4512,7 @@ qemuDomainGetVcpusFlags(virDomainPtr dom, unsigned int flags)
     int ret = -1;
     virCapsPtr caps = NULL;
     qemuAgentCPUInfoPtr cpuinfo = NULL;
-    int ncpuinfo;
+    int ncpuinfo = -1;
     int i;
 
     virCheckFlags(VIR_DOMAIN_AFFECT_LIVE |