This commit fixes an issue where migration is failing in the load phase
because of a false alarm about data unavailability.

Following is the error received when the amount of data to be transferred
exceeds the default buffer size setup by G_BUFFERED_INPUT_STREAM(4KiB),
even when the maximum data size supported by this backend is 1MiB
(DBUS_VMSTATE_SIZE_LIMIT):

  dbus_vmstate_post_load: Invalid vmstate size: 4364
  qemu-kvm: error while loading state for instance 0x0 of device 'dbus-vmstate/dbus-vmstate'

This commit sets the size of the input stream buffer used during load to
DBUS_VMSTATE_SIZE_LIMIT which is the maximum amount of data a helper can
send during save phase.
Secondly, this commit makes sure that the input stream buffer is loaded before
checking the size of the data available in it, rectifying the false alarm about
data unavailability.

Fixes: 5010cec2 ("Add dbus-vmstate object")
Signed-off-by: Priyankar Jain <priyankar.jain@nutanix.com>
Message-Id: <cdaad4718e62bf22fd5e93ef3e252de20da5c17c.1612273156.git.priyankar.jain@nutanix.com>
[ Modified printf format for gsize ]
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>