block/sheepdog: fix argument passed to qemu_strtoul()

block/sheepdog: fix argument passed to qemu_strtoul()

The function qemu_strtoul() reads 'unsigned long' sized data,
which is larger than uint32_t on 64-bit machines.

Even though the snap_id field in the header is 32-bits, we must
accommodate the full size in qemu_strtoul().

This patch also adds more meaningful error handling to the
qemu_strtoul() call, and subsequent results.

Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Message-id: e56fc50abedd9a112e0683342c8eafda063cd2f9.1456935548.git.jcody@redhat.com