qga: add qga_open_cloexec() helper

qga: add qga_open_cloexec() helper

QGA calls qemu_open_old() in various places. Calling qemu_open() instead
isn't a great alternative, as it has special "/dev/fdset" handling and
depends on QEMU internal monitor data structures.

Instead, provide a simple helper for QGA needs, with Error* support. The
following patches will make use of it.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20220525144140.591926-6-marcandre.lureau@redhat.com>