acpi: Simplify printing to dynamic string

acpi: Simplify printing to dynamic string

build_append_namestringv() and aml_string() first calculate the
resulting string's length with vsnprintf(NULL, ...), then allocate,
then print for real.  Simply use g_strdup_vprintf() or g_vasprintf()
instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>