libxl: add vmemrange to libxl__domain_build_state

libxl: add vmemrange to libxl__domain_build_state

A vnode consists of one or more vmemranges (virtual memory range).  One
example of multiple vmemranges is that there is a hole in one vnode.

Currently we haven't exported vmemrange interface to libxl user.
Vmemranges are generated during domain build, so we have relevant
structures in domain build state.

Later if we discover we need to export the interface, those structures
can be moved to libxl_domain_build_info as well.

These new fields (along with other fields in that struct) are set to 0
at start of day so we don't need to explicitly initialise them. A
following patch which introduces an independent checking function will
need to access these fields. I don't feel very comfortable squashing
this change into that one so I didn't use a single commit.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Dario Faggioli <dario.faggioli@citrix.com>
Cc: Elena Ufimtseva <ufimtseva@gmail.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>