virt.guest_agent: Fix guest agent exception initiation problem

There are some wrong format of in guest agent exceptions
definition, This patch fixes these problem.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virt.guest_agent: Update qemu guest agent __init__ function

Since we updated the initiation arguments, guest agent module
needs updates as well.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
qemu.tests.qemu_guest_agent: Add a subtest suspend for qemu guest agent

This 'suspend' subtest run guest s3/s4 test with
qemu guest agent commands, to verify guest agent's
suspend commands.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virt.guest_agent: Fix VAgentCmdError exception

This exception class raises another TypeError when the
'args' argument is NoneType. It looks that this class
used a reserved argument in Exception class.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virt.guest_agent: Add a method for sync qemu guest agent

The guest agent doesn't provide a command id in its response,
so we have to send 'guest-sync' cmd by ourselves to keep the
socket synced.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>