Add context as parameter for two libvirt APIs

When finish_revert_migration is called, the caller of
finish_revert_migration already includes context as a parameter,
but finish_revert_migration did not reuse this parameter and still
re-generate the context inside finish_revert_migration, we should
add context as a new parameter for finish_revert_migration so that
the functions inside of it can reuse context when needed.

For the function _create_domain_and_network in libvirt/driver.py,
it set context as an optional parameter, but context isn't really
an optional parameter for this method. We should always pass a
context down because it might be needed somewhere inside
_create_domain_and_network().

The patch includes two parts:
1) Add context as a new parameter for finish_revert_migration
2) Set context as a required parameter for _create_domain_and_network

Change-Id: I72d131fc8dce903400e3080d51ba0fc0a1320751
Closes-Bug: #1251261
Merge "Add context as parameter for two libvirt APIs"