[TPM] Fix an occasional problem when doing local migration due to the xenstore's

[TPM] Fix an occasional problem when doing local migration due to the xenstore's
'instance' variable not being initialized at the time when it's driver
counterpart is accessed. That variable's content was used to initialize the
tpmif structure. Instead, now a pointer to the structure is passed whose
'instance' variable will be initialized before the domain is resumed.

Signed-off-by: Stefan Berger <stefanb@us.ibm.com>