blktap2: fix save/restore/migration

blktap2: fix save/restore/migration

blktap2 devices use a regular 'phy' vbd blkback backend, causing
Blktap2Controller to trample the devices' parameters. This causes
problems with save/restore and managed domains, among other
things. This patch modifies Blktap2Controller to store both the vbd
and tap2 parameters in xenstore, and stops it from trampling the
device's config on device creation.

 * store blktap2 parameters in xenstore
 * restore blktap2 device config to original state once the underlying
 * vbd
   device is created (this fixes managed domains)
 * use blktap2 parameters rather than vbd parameters when building
 * blktap2
   device configurations
 * remove blktap2 specific code from XendConfig

Signed-off-by: Ryan O'Connor <rjo@cs.ubc.ca>