xen: do not use '%ms' scanf specifier

xen: do not use '%ms' scanf specifier

The 'm' parameter used to request auto-allocation of the destination variable
is not supported on FreeBSD, and as such leads to failures to parse.

What's more, the current usage of '%ms' with xs_node_scanf() is pointless, as
it just leads to a double allocation of the same string.  Instead use
xs_node_read() to read the whole xenstore node.

Fixes: a783f8ad4ec9 ('xen: add a mechanism to automatically create XenDevice-s...')
Fixes: 9b7737469080 ('hw/xen: update Xen console to XenDevice model')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>