migration: file URI offset

migration: file URI offset

Allow an offset option to be specified as part of the file URI, in
the form "file:filename,offset=offset", where offset accepts the common
size suffixes, or the 0x prefix, but not both.  Migration data is written
to and read from the file starting at offset.  If unspecified, it defaults
to 0.

This is needed by libvirt to store its own data at the head of the file.

Suggested-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <1694182931-61390-3-git-send-email-steven.sistare@oracle.com>