virttest.qemu_devices: Set 'serial' parameter only when qemu supports it

Most qemu versions supports setting -drive serial=xxx, but only latest
ones supports setting serial using -device serial=xxx.
With this patch we first check, if qemu supports -device serial=xxx
and in case it does not, we skip it (it's set by -drive anyway).

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
virttest.qemu_devices: Cache the last execute_qemu output

As this function might be called multiple times, cache the last
output and reuse it.

For example multi_disk should benefit from this as it adds hundreds
disks and each one of them gathers informations about supported
options.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>