KVM: Fix ERROR: do not initialise statics to 0 or NULL in kvm_main.c

ERROR: do not initialise statics to 0 or NULL
+static int kvm_usage_count = 0;

The kvm_usage_count will be placed to .bss segment when linking, so
not need to set it to 0 here obviously.

This patch fixes this ERROR to reduce noise when checking new patches
in kvm_main.c.

Signed-off-by: Xiubo Li <lixiubo@cmss.chinamobile.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>