xl: Remove some duplicated boilerplate. (Improves logging slightly.)

xl: Remove some duplicated boilerplate. (Improves logging slightly.)

We remove six lines of boilerplate from the top of each function, and
instead have a single struct libxl_ctx which is initialised once at
the top of main.

Likewise we wrap domain_qualifier_to_domid in a new function
find_domain, which does the error handling, and stores the domid and
the specified name (if applicable).

This reduces the size of xl.c by 7% (!)

As a beneficial side effect, the earlier call to libxl_ctx_set_log in
main makes some lost messages appear.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>