block: Make 'bytes' param of blk_{pread,pwrite}() an int64_t

For consistency with other I/O functions, and in preparation to
implement them using generated_co_wrapper.

Signed-off-by: Alberto Faria <afaria@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220705161527.1054072-5-afaria@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>