qga: replace qemu_open_old() with qga_open_cloexec()

qemu_open_old() uses qemu_open_internal() which handles special
"/dev/fdset/" path for monitor fd sets, set CLOEXEC, and uses Error
reporting (and some O_DIRECT special error casing).

The monitor fdset handling is unnecessary for qga, use
qga_open_cloexec() instead.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Message-Id: <20220525144140.591926-9-marcandre.lureau@redhat.com>