libxc: use an explicit check for PV MSRs in xc_domain_save()

libxc: use an explicit check for PV MSRs in xc_domain_save()

Migrating PV domains using MSRs is not supported.  This uses the new
XEN_DOMCTL_get_vcpu_msrs and will fail the migration with an explicit error.

This is an improvement upon the current failure of
  "No extended context for VCPUxx (ENOBUFS)"

Support for migrating PV domains which are using MSRs will be included in the
migration v2 work.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>