migration/savevm: wrap into qemu_loadvm_state_header()

migration/savevm: wrap into qemu_loadvm_state_header()

On source side, we have qemu_savevm_state_header() to send related data,
while on the receiving side those steps are scattered in
qemu_loadvm_state().

This patch wrap those related steps into qemu_loadvm_state_header() to
make it friendly to read.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Message-Id: <20190424004700.12766-5-richardw.yang@linux.intel.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>