Remus: Make xc_domain_restore loop until the fd is closed.

Remus: Make xc_domain_restore loop until the fd is closed.

The tail containing the final PFN table, VCPU contexts and
shared_info_page is buffered, then the read loop is restarted.
After the first pass, incoming pages are buffered until the next tail
is read, completing a new consistent checkpoint. At this point, the
memory changes are applied and the loop begins again. When the fd read
fails, the tail buffer is processed.

Signed-off-by: Brendan Cully <brendan@cs.ubc.ca>