libxl: implement virDomainMemorystats

libxl: implement virDomainMemorystats

Introduce support for domainMemoryStats API call, which
consequently enables the use of `virsh dommemstat` command to
query for memory statistics of a domain. We support
the following statistics: balloon info, available and currently
in use. swap-in, swap-out, major-faults, minor-faults require
cooperation of the guest and thus currently not supported.

We build on the data returned from libxl_domain_info and deliver
it in the virDomainMemoryStat format.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>