Require a semicolon for VIR_ONCE_GLOBAL_INIT calls

Require a semicolon for VIR_ONCE_GLOBAL_INIT calls

Missing semicolon at the end of macros can confuse some analyzers
(like cppcheck <filename>). VIR_ONCE_GLOBAL_INIT is almost
exclusively called without an ending semicolon, but let's
standardize on using one like the other macros.

Add a dummy struct definition at the end of the macro, so
the compiler will require callers to add a semicolon.

Reviewed-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>