qga/command: use QEMU atomic primitives

qga/command: use QEMU atomic primitives

The guest client's use of the glib's g_atomic primitives causes newer
GCC's to barf when built on Travis. As QEMU has its own primitives with
well understood semantics we might as well use them.

The use of atomics was a little inconsistent so I've also ensure the
values are correctly set with atomic primitives at the same time.

I also made the usage of bool consistent while I was at it.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20160930213106.20186-12-alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>