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 <[email protected]>
Message-Id: <20190424004700[email protected]>
Reviewed-by: Daniel Henrique Barboza <[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>