CA-114341: check return value of LVHD plugins

Initially the return value of the call to the plugin was checked (when
the plugin was called via the xe command), and when xe was replaced the
check was removed. It is unclear why this happened (no justification in
the commit messages), so it might be an omission.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Germano Percossi <germano.percossi@citrix.com>
CA-114341: GC: acquire VDI lock non-blocking

The GC needs to acquire the SR and VDI locks in the reverse order
VDI.activate/deactivate do, which leads to temporary deadlock that is
resolved when the VDI.activate/deactivate give up. This patch makes the
GC to simply try to acquire the VDI lock, if it fails to do so it skips
the VDI, and that leads to the GC releasing the SR lock.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Germano Percossi <germano.percossi@citrix.com>
Merge CA-114341

Signed-off-by: Germano Percossi <germano.percossi@citrix.com>

GitHub: closes #84 on xapi-project/sm