xen: don't let bogus packets trigger over-allocation and segfault

xen: don't let bogus packets trigger over-allocation and segfault

* src/xen/proxy_internal.c (xenProxyDomainDumpXML): An invalid packet
could include a too-large "ans.len" value, which would make us allocate
too much memory and then copy data from beyond the end of "ans",
possibly evoking a segfault.  Ensure that the value we use is no
larger than the remaining portion of "ans".
Also, change unnecessary memmove to memcpy (src and dest obviously
do not overlap, so no need to use memmove).
(xenProxyDomainGetOSType): Likewise.
(xenProxyGetCapabilities): Likewise.