virsh: Introduce 'await' command for waiting until target domain state is reached

virsh: Introduce 'await' command for waiting until target domain state is reached

The new command is meant as syntax sugar for event handling which blocks
virsh until the requested state condition is reached.

The initial implementation adds a condition 'domain-inactive' returning
if the domain is/becomes inactive for whatever reason.

This command is useful for simple scripts e.g. for debugging libvirt
when it allows responding to target state in shell without the need to
fuss too much with polling or writing handlers around 'virsh event'.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>