xenpaging: reduce number of qemu cache flushes

xenpaging: reduce number of qemu cache flushes

Currently the command to flush the qemu cache is called alot if there
are no more pages to evict. This causes churn in the logfiles, and qemu
can not release more pages anyway since the last command.

Fix this by remembering the current number of paged-out gfns, if this
number did not change since the last flush command then sending another
new flush command will not free any more gfns.

Remove return code from xenpaging_mem_paging_flush_ioemu_cache() since
errors do not matter, and will be handled elsewhere. Also failure to
send the flush command is not fatal.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Committed-by: Ian Jackson <ian.jackson.citrix.com>