CA-94406: VDI reset only on epoch begin

This resolves a bug where VDIs were reset after migration, because the act of
plugging a VBD will reset the VDI. A corresponding change in the storage
managers moves the reset_leaf call (which resets VDIs) to the SR command
"vdi_epoch_begin", which was recently added to Xapi's SMAPIv2.

This commit just adds the plumbing necessary to call epoch_begin and epoch_end
on VDIs in the storage layer. The corresponding commit in sm.hg adds those
commands to the storage layer, and also moves the reset_leaf call into
vdi_epoch_begin.

Signed-off-by: Mike McClurg <mike.mcclurg@citrix.com>