libxl: Provide more formal libxl__ctx_lock and _unlock

libxl: Provide more formal libxl__ctx_lock and _unlock

Previously the only official interface for the ctx lock was the
CTX_LOCK and CTX_UNLOCK convenience macros, which assume and use "ctx"
from the surrounding scope.

Instead, provide libxl__ctx_lock and _unlock functions which can be
used by these convenience macros, and other callers who have
nonstandard requirements.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>