Add API to change qemu agent response timeout

Some layered products such as oVirt have requested a way to avoid being
blocked by guest agent commands when querying a loaded vm. For example,
many guest agent commands are polled periodically to monitor changes,
and rather than blocking the calling process, they'd prefer to simply
time out when an agent query is taking too long.

This patch adds a way for the user to specify a custom agent timeout
that is applied to all agent commands.

One special case to note here is the 'guest-sync' command. 'guest-sync'
is issued internally prior to calling any other command. (For example,
when libvirt wants to call 'guest-get-fsinfo', we first call
'guest-sync' and then call 'guest-get-fsinfo').

Previously, the 'guest-sync' command used a 5-second timeout
(VIR_DOMAIN_QEMU_AGENT_COMMAND_DEFAULT), whereas the actual command that
followed always blocked indefinitely
(VIR_DOMAIN_QEMU_AGENT_COMMAND_BLOCK). As part of this patch, if a
custom timeout is specified that is shorter than
5 seconds,  this new timeout is also used for 'guest-sync'. If there is
no custom timeout or if the custom timeout is longer than 5 seconds, we
will continue to use the 5-second timeout.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>