util: replace atomic ops impls with g_atomic_int*

util: replace atomic ops impls with g_atomic_int*

Libvirt's original atomic ops impls were largely copied
from GLib's code at the time. The only API difference
was that libvirt's virAtomicIntInc() would return a
value, but g_atomic_int_inc was void. We thus use
g_atomic_int_add(v, 1) instead, though this means
virAtomicIntInc() now returns the original value,
instead of the new value.

This rewrites libvirt's impl in terms of g_atomic_int*
as a short term conversion. The key motivation was to
quickly eliminate use of GNULIB's verify_expr() macro
which is not a direct match for G_STATIC_ASSERT_EXPR.
Long term all the callers should be updated to use
g_atomic_int* directly.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>