migration: Fix regression for xenfv and pc,accel=xen machine.

This fix migration from the same QEMU version and from previous QEMU
version.

>From the global state section, we don't need runstate with Xen. Right now,
the way the Xen toolstack knows when QEMU is ready is when QEMU reach
"running" runstate.

The configuration section and the section footers are not going to be
present in previous version of QEMU with xenfv machine, so we skip them.

The Xen toolstack libxenlight does not specify a particular version of the
'pc' machine, so migration from older version of QEMU used by Xen to newer
one would break due to missing "configuration" section and section footers.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>