tools: add --maxmem parameter to init-xenstore-domain

tools: add --maxmem parameter to init-xenstore-domain

Add a parameter to specify the maximum memory size of the xenstore
domain. In case the xenstore domain supports ballooning it will be
capable to adjust its own size according to its memory needs.

The maximum memory size can be specified as an absolute value in
MiB, as a fraction of the host's memory, or as a combination of
both (the maximum of the absolute and the fraction value):

--maxmem <m>             maxmem is <m> MiB
--maxmem <a>/<b>         maxmem is hostmem * a / b
--maxmem <m>:<a>/<b>     maxmem is max(<m> MiB, hostmem * a / b)

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>