exec: Fix qemu_ram_block_from_host for Xen

Since f615f396 (exec: remove ram_addr argument from
qemu_ram_block_from_host), migration under Xen is likely to fail, with a
SEGV of QEMU. But the commit only reveal a bug with the calculation of
the offset value in qemu_ram_block_from_host().

This patch calculates the offset from the ram_addr as
qemu_ram_addr_from_host() will later calculate the ram_addr from the
offset.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>