tools/xenconsoled: Possibly perform repeated xc_readconsolering() hypercalls

tools/xenconsoled: Possibly perform repeated xc_readconsolering() hypercalls

The size of the Xen console ring is runtime configurable, so the statically
sized 16k buffer is not necessarily sufficient.  Make repeated
xc_readconsolering() hypercalls while Xen managed to completely fill the
provided buffer.

Also, change the buffer to being static to save on stack space.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>