seqlock: introduce read-write seqlock

Seqlock implementation for QEMU. Usage idiom

reader:
    do {
        start = seqlock_read_begin(&sl);
        ...
    } while (seqlock_read_retry(&sl, start));

writer:
    seqlock_write_lock(&sl);
    ...
    seqlock_write_unlock(&sl);

initialization:
    seqlock_init(QemuSeqLock *sl, QemuMutex *mutex)

    mutex could be NULL if the caller will provide its own protection
    for concurrent write sides (typically using the BQL).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
