Change xm dmesg -c so that it prints out the current buffer as well as clearing

Change xm dmesg -c so that it prints out the current buffer as well as clearing
it.  This avoids a race so that you don't lose messages, matches the behaviour
of the hypercall, and the Unix dmesg command.

Change the Xen-API binding to use a separate function host.dmesg_clear rather
than a flag on host.dmesg.

Signed-off-by: Ewan Mellor <ewan@xensource.com>